I will put more writing on this page, some over the top of the undercollage, to take it more into the background.
Here I used torn edges so they would blend in more. Either way. Do different page backgrounds differently. Try out stuff. It's fun.
Glue them down well with acrylic matte medium (be generous). Stick glue won't keep its 'stick' when you paint over the top. If you smooth the papers down with a ruler or credit card while still drying you will have less bubbling.
Then paint over the top with gesso. For this background I used black gesso, with the first one I used white (forgot to take a photo of the white one.) You can wipe off some of the gesso if you want the undercollage to show more. But it is intended to be a background. I usually put more collage over the top.
Here is the beginning of some collage and drawing over the first undercollage.
You can see how the magazine images were knocked back with the white gesso. Some of the images I completely covered, others I wiped back again. I then painted over the gesso before I added the collage and drawing.