It's super easy. Buy large sheets of coroplast (corrugated plastic) cut them to size and zip tie them to the holes in your frame. The main idea is to smooth out the turbulent areas such as under the engine bay and below the trunk/spare tire well. Don't exceed a 15* angle from horizontal and you'll be fine. Check my Anal Probe thread in the Ecomodder Central section for some pics, and good luck!
