1. Refer to Part 3 of 7 in
this friendship guide.
@Sharksheep's guide is also really well-written!
2. If you go to Harv's Island for Photopia, you can enter decorating mode inside Harv's studio then call your current villagers to act as models in the photoshoot. If you have Amiibos, I think you can also invite them to act as models, too, though I haven't tried this myself. When you return to your island and visit the Nook Stop/use the Nook Shopping app, there will be a tab labeled
Posters, and the posters can be ordered thereafter.