Since you're modifying an existing PrC instead of creating a new one from scratch, things will be slightly easier.
You'll want to modify the following 2DAs:
- CLASSES.2DA
- CLS_BFEAT_ARCHER.2DA
- CLS_FEAT_ARCHER.2DA
- CLS_FEATMAP_ARCHER.2DA
You'll also have to create the following 2DA, which will define the bonus spells per level:
The AA class in Pathfinder includes spell progression, so you could model it off that.
If you want to create separate spell progression feats just for the Arcane Archer (for wizard, bard, and sorcerer) then you'll also have to edit FEAT.2DA. If you're only adding spell progression for your own benefit, then you can recycle the feats from another PrC (like Eldritch Knight). They'll work just the same, except their descriptions won't say 'Arcane Archer' in them. By recycling other PrC feats you also avoid having to modify the dialog.tlk file.
When in doubt, look at one of the other PrCs with spell progression (like the Eldritch Knight) and figure out how the 2DAs work. The first post in this thread should point you in the right direction.