Pick a design, or an art-theme for your collage. This is for the deeper thinker. Think "color", as in shades of one color-tone, or one or more color-tones, or, think "consistency", and that would be one that expresses a particular message, be it intellectual property, or certain types of pictures, say, cartoons, or Postage Stamps, or scraps of fabric, or names, or alphabets, or flowers, or animals, or landscapes, or family pix, etc. or any combination thereof.
One time I made a postage stamp "design collage" on a tray. I applied many coates of Mod Podge afterwards, to give it a finished surface.
Another time I collected pix of pretty angels' wings from all different sources, put them all in their own folder (we're talkin hard copy, of course) and when the time was right I assembled them all into one "design collage"; I believe it was on a decoupage plaque I picked up at a thrift store.