We cut the thick foam board into strips to line the bottom indented space of the frame (right next to the glass, but leaving a space between it and the glass), and put another strip along the back of the frame just below the first strip. We hot glued these on with the foam aiming up. Before glueing on the front strip, I attached some brown scrap material to mask the white foam board that faced the glass. We used these strips to poke the grasses and weeds into and hot glued them in place as needed. Sometimes pre-poking a hole with a toothpick was helpful for inserting finer plant stems.
When designing the frame we propped it up on our table near a front window so we could work on both sides of it. It was easier to arrange it with one person on each side of the frame. We added little nature finds (milkweed seeds, feathers, seed pods, etc.) in the little well created in front of the foam board strip at the bottom. We all liked the way this turned out.