I have done a number of collages, using a number of methods, Susan Carlson's being one of them.
I've done with and without the tulle. If you get the right colour, it can really add to your collage. You can also add the tulle in different colours to different areas....the background in one colour and then the chicken covered in another colour, as an example. It doesn't all have to be one.
he horse in this collage is mostly in grey tulle, but his lighter areas are in white tulle. He was collaged with Steam a Seam Lite 2, but with 1/4" squares of fabric, so I felt the need to put tulle over top, with so many tiny pieces. Quilted on a Tiara.
This dog collage was done with glue, but I didn't do tulle. I only quilted around the dog and lines in the background. It's held up to my daughter moving numerous times, quite well. I did take quite some time to be sure all the pieces and edges were glued down. I even went over it lightly with a lint roller to see if I could lift any edges.
Watson