The way I did mine was with wood furring strips. I placed a thin drywall knife inbetween them while fastening them to the soffit. Later when I did the fabric I followed the same procedure ChinaDog did, except I stuffed the fabric into the thin gap with the same drywall knife used to create the gap. You can get different thickness of furring strips with Fabricmate your limited to 1/2 and 1" thickness.
"Anyone who lives within their means suffers from a lack of imagination."