When I introduce characters I always feel like I take to much time describing how they look. What is a good way to let the readers know what the character(s) look like without just saying it straight out?
You can usually pick a single detail, or maybe two, that kind of represent the character, and leave the rest for the reader to fill in. You can also combine it with actions that are useful to the plot projection and details that help with characterization. At some point you need to be able to let go of total control over your readers' experiences/impressions. For example: The door opened, then, a too-thin girl with rainbow hair and a lot of blurry eye makeup staring out at them as if she’d just woken up. Or maybe as if she were stoned. That's the only description I give of a new secondary character in my current series. It's not enough to create a vivid picture of the exact character I see, but I don't think it needs to do that. It's just enough for my readers to get some idea of what she looks like, and they can each fill in the details according to their own experiences.
Yeah, I generally agree with this. There's absolutely nothing wrong with stretching out details over a long period, or even just leaving them out if they aren't a big deal. When you are introducing a character just go with the most striking features of them, ones that would make them stand out to whoever is looking at them. If they have piercing blue eyes or a purple mohawk then yeah, mention it because someone is probably going to talk about it. But for the rest of it? Eh. Drop the broad strokes, add the important details, then fill stuff in as it comes up. In my stuff I kinda go even further - I (semi-intentionally; I don't like writing description) treat any descriptive like it's Chekov's Gun. If it get's mentioned it's because it'll come back with some meaning. Not always tonnes of meaning, but it has to come back and be relevent somehow at some point. That doesn't mean I don't write description. But if stuff just doesn't matter then I don't bother even on main characters. Shapes of ears and noses and eyebrows I just don't care about; you can't express character stuff through them so I don't care and mostly I figure the readers don't either. When a guy is just a corporate suit then I'll just describe the suit. If a girl is pretty but not important to the plot then I just say she's pretty and leave it at that. And sure, that's just me and I'm a crazy person (not because of this, but I am) so maybe that doesn't fit for you but you'd be surprised how little it really changes to make details earn their keep like that.
As was indicated above, you don't need a lot for a reader to create an image in their mind's eye. Being too controlling--giving the reader exacting descriptions slows the reading experience down. Subtle things like, I had to look up into his green eyes, indicates he's taller than the anchor character, the one describing. Check out a few of your favorite authors. See how they brought a new character into the story. That will offer you solid ideas of what has worked. Then apply what they did to your own story/characters and writing style.
I think this is the major point. We as writers have a tendency towards over-describing because we want to make sure readers see characters exactly as we do. It matters a lot to us, of course it does. We like our characters, have an understandable attachment to them, we want to make sure they are properly represented. And readers like to see what we're describing but that should be secondary to telling the story. Absolutely describe your characters and paint your world but don't do so at the expense of actually telling your story.
Totally agree with with @LostThePlot on this one. Try not to go overboard on new characters otherwise they take away from the story.
I see three ways of looking at this. The first, and the one I've heard most often, is that you should establish something early if it determines what the reader will expect. If a character wears their hair in a bright pink mohawk, that gives a pretty good idea of what kind of person they are, and you might not need to give as many other details to provide a mental image of them. The second is that you should establish whatever the reader will most need to know for later. If you're mentioning in chapter 10 that the character got her facial scar from the villain, you probably ought to let readers know in chapter 1 that she has a facial scar. The third is to specify whatever the reader would least expect given context, preventing them from creating a false mental image that they'll later have to revise. For instance, I once confused a beta reader by taking too long to establish that farmers from a cold forested region were dark-skinned. If they'd been living in a desert, I might not have needed to say that at all. Naturally, this makes it really tricky to give a good description of a culture or species you've invented for your story. I've never quite gotten the knack of that. Addendum one: you can get a lot of mileage out of other characters' reactions, especially if your story involves a conflict between classes or cultures. TWErvin2 mentioned having one character look up to indicate another character is taller. I've also seen authors who wrote about racism without ever directly stating that the viewpoint character was black, just showing how other characters reacted to them in racist ways. Addendum two: some characters don't need a specific mental image, especially protagonists. Just establish what's necessary to show how they fit into the world. (For instance, one of my protagonists fights in close quarters, so all I mention is that she's physically fit and wears her hair short.)
I think all three of @Feo Takahari's points are excellent. One thing I will add is that you can't please all readers. I prefer very light descriptions, like most people on this thread it seems, but some readers get annoyed when they aren't told how to picture a character. We all have arbitrary preferences and you simply can't cater to all of them.
That's definitely worth remembering. At the end of the day we are all just talking about preferences. I happen to think that details can be chopped damn near whole-sale; and in fact I'd go beyond that and say I think it's actively bad writing to over-describe buuuuuut what do I know? I know people who genuinely think LoTR needs more Tom Bombadiel (just typing that gave me a migraine). People are crazy and hold crazy opinions. On this case I don't think I'm being all that crazy but whatever point on the spectrum feels good to you someone is going to tell you why you're wrong and that makes you a dreadful writer. It's still good general purpose advice to, if all else fails, err on the side of cutting stuff that isn't really doing anything but it's up to you exactly how much describing your characters nostril hairs does. I'm not going to tell you that couldn't be interesting (or at least funny) and if you think it is then keep it.
I typically let my characters actions speak for them. I tend to write in a very narrative, event based style based around character interactions rather than the world around them. So every action they have in regards to the main character, I give a little description along the way: "She walked through the revolving door, and John couldn't help but notice a beautiful set of green eyes," Things like that. Just one step at a time, and always inspired by just a random action or meeting, much like real life.
I pretty much agree with the general consensus: take the most pertinent details and don't go overboard. I tend to think about what I first think of when I picture my friends and family and use that as a guide. One of my friends had long curly red hair when I met him, and where I live that's fairly uncommon, so if my hometown is the 'setting' then that's the remarkable detail. Another friend is really tall but the thing that struck me about him is that he has long, slender fingers - which I noticed because he talks with his hands, so it stood out. So picture your character, entering the scene however they do, and see what sticks out. From there you can sprinkle in other details as you go. In something I'm currently procrastinating on finishing, the first real description of a character is that he's barefoot, because he's trying to be very quiet in an echoey stone chamber. A few lines later I mention his eyes flashing yellow in the light, indicating that he's not all human. 1300w later I finally mention his hair color and style because it gets messed up. But that's just kind of my style - I mostly prefer to mention most things only as they become relevant.
I like to give a good more or less immediate description, but I try to keep it short and meaningful. Adding a sense of who they are as a person and what their introduction mean to others in the way I describe their appearance.. One of the things I'm trying to improve one is interesting way to mention details, which helps with that. So I've been implementing techniques like describing them griping a cane tightly instead of just saying they have it. Or a character being struck by their eyes rather than just observe their beautiful eyes. I think that's good enough that it compensates for the description dump.