Here are the directions on my wonder under.
1. Place rough side against wrong side of fabric. Press for 5-8 sec with hot, dry iron. Let cool, draw desired shape onto paper backing, then cut.
2. Gently peel off paper backing. Position fusible fabric, web side down, on the project. Cover with DAMP press cloth. With iron at WOOL setting, press FIRMLY for 10-15 sec. (Heavier fabrics = more time.) Repeat, lifting and overlapping iron until all fabric is fused. Premove press cloth and iron fabric to eliminate excess moisture.
TIP: Use your damp press cloth as a temp. and timing guide. After 10 sec, press cloth should be dry. If not, raise iron temp or hold iron in place a few sec longer.
I haven't used Wonder Under in a while so I don't have any other suggestions. I hope maybe this helps. Good Luck.