Leave the medallions their flat white color or paint them the color of the wall underneath
Experiment with different arrangements by using trace paper cutouts of the shapes and tape those to the wall.
Remove the backing from a 3m command strip and apply it to the back of a medallion centered near the top. Then remove the other backing strip and press it firmly against the wall or use adhesive caulk.
Alternate: You can attach medallions using any picture hanging method and this same technique works with old picture frames
For a “blind embossed” look, paint medallions the same color as the wall
For large areas, trace medallions with tracing paper and tape to wall to decide layout
3m command strips won’t damage the wall and you can remove them
Use adhesive caulk for a more permanent fixture; try loctite 2 in 1 seal & bond, loctiteproducts.com
Other simple picture hanging techniques will work depending how permanent you want them
Also try this technique with a variety of picture frames
Check out more fun DIY projects at the Home & Family Pinterest Page