I'm writing a fantasy series and I plan to make one of the characters meek and simple-minded at first, but how would I go about it?
The easiest way is to not show these as being positive qualities. If he gets taken advantage of a lot or is easily lead then we'll sympathize with him because people are, well, taking advantage of him being nice. That's pretty clear cut and easily done; the challenge would be in making these qualities in him seem like a good thing. It's sad but true that cynicism is normal in humans as is selfishness and someone who is pathologically altruistic is someone we just don't want to root for. Even in the new testament with all it's blessed are the meek messages it's not telling you to give away all your possessions and live in a cave. It's saying that we should be as charitable as we can without destroying our families future. So if you want his meekness to be a bad thing (or at least not just a good thing) then no problem. If you want him to look virtuous and whatever then that'll be a lot harder to get people invested in.
The same way you'd write any character - you create a scene that will show the reader their qualities. If a character is an angry or jealous person - you create a scene or scenes to showcase that. Same deal here. Just think of a setting and a situation that will help reveal the character's meekness - does he let himself get bullied or coerced into things? Is his opinion of no value? Are their times when he doesn't stick up for himself or his opinion? What can you have him do to show he is simple minded?