If you want complete control over the aesthetic of your collage, the Instagram "Stickers" function is perhaps the best way to do it. Stickers might be made out of photographs, photos, or pre-made artwork.
When you go into "Your Story" and select "Stickers," you can choose and customise your background as well as select or create photographs for the collage. The biggest benefit of this method is that there are no limits to the number of photos/images/stickers you can include in your collage. You can rotate, resize, move, and overlap them as much as you want. You can create some really cool collages! So this is how to make a collage on instagram