Q1: Normal format for manuscripts calls for one inch margin on all four sides. However, should one add a gutter? Q2: Should one use mirror pages im the manuscript? In this case, the gutter would be on the right of the first page and on the left of the second page. Q3: Is it now correct to submit using Courier New font?
From what I've read.... Q1, No need so don't bother. No agent is going to be bothered if you didn't Q2, N/A see above. Q3, Courier font smacks of ex script writer having a go at a novel. Once it was a preferred font - mostly in the US. You won't go wrong with NTR.
Q1. No. Manuscripts are submitted without a binding. Q2. No, manuscripts in hard copy are printed on one side only. In soft copy they should be ready-to-print, also for single sided printing. Q3. Yes, unless the publisher requests a specific font. Most publishers prefer a monospace (i.e. not proportional spaced) serif font, and Courier or Courier New are preferred, or at least acceptable, to most. Fixed sized fonts make it easier to spot typos.
