Add support to large collage to prevent warping and tearing

For your current painting assemblage there is very little you can do. The paints will have warped the paper, and the glue might have done the same.
You can glue the piece unto hardboard or another rigid surface (using, like virolino mentioned, a non-aqueous glue, such as glue-sticks, like Pritt's - be careful not to get too close to the paints, though, or they might leak), or frame it behind glass.

For your next project, I suggest using stronger paper, intended for use with paints, and taping it to a rigid surface using aquarel tape (a.k.a. water-activated paper tape - here is a video explaining the procedure, although I'd like to add that it's best to wait a little with applying the tape after wetting it; it will first shrink, and then loosen up again, and that moment is the right moment to apply it).
After it has dried, you can remove the tape or cut out the paper, and it will be completely flat.

Paper has the habit of becoming wavy when wet with water (other liquids too).

So the first thing you might want to try is ti use a non-watery glue, but still suitable for paper.

For the next projects, you might want to investigate about the types of paper available, to choose one more suitable for what you intend to do.

During curing, make sure to apply pressure to the area with the glue - on a flat surface, of course. Worst case, turn a kitchen table up-side-down, place it over the collage, and then add some serious weight over (now inside) it. The floor has to be flat also, or use another table as a support.

Another hint: use a non-stretchable support for the collage, e.g. glass-fiber mesh. You can even build a sandwich: cardboard + glass-fiber mesh + collage (for some added strength).