I've created a few collages on cardboard for the walls of my room. Here is the process for one of them as GIF with steps below (click it to replay it).
1 - I sweeped up my room first to prevent dirt from getting on the adhesive tape.
2 - Next I chose sealife focused photography books I bought used for a few euros.
3 - Then I searched the photos I find fitting. I looked for underwater shots for this one.
4 - Cutting out the pages I liked I tried out putting them together on the cardboard.
5 - I took care of the overall flow and the edges as far as possible.
6 - To cut off pieces and torn edges I used two woodboards with straight edges and a cutter.
7 - The adhesive tape came into play as soon as I knew how I definitely wanted to begin.
8 - I fixed every piece with a small piece of tape when I've found the right place. No need to fix them completely, since newer ones will overlap the old ones.
9 - As soon as everything is fixed to its place I use the adhesive tape to fix all edges, as you can see on the reflection. This makes it all more flat and prevents it from easily being damaged.