I will be using watercolor as my surface for this project. Watercolor paper works well for collage because it is made to get really wet. A cradled and gessoed wood panel would be even better.
It takes some practice to get a piece of collage to glue flat without any buckles. Paper is very porous so when you get it wet with glue it will expand. If you only put glue on one side of the paper the paper will expand unevening and if you try to lay it down on your collage it will buckle. The way to avoid this is to get both sides of the collage element wet with glue AND get the surface you are putting the collage element on wet with glue. This way everything is completely expanded and will shrink and tighten as it dries.