Hey Scott - there's dozens of ways you might do this but why not try this method and see if it works for you. Sorry that it's very step-by-step, but it's easier for me to explain this way :
First, I assume you have a nylon tape loop sewn at each end of the "ridge" of your basha. Maybe one in the middle of the tarp too - to stop the middle of the ridge sagging. If you don't, then read on anyway and see my comments at the end.
1. Thread your ridge cord through each tape loop in turn so that the basha hangs underneath the ridge cord from the 2 or 3 tape loops.
2. Hanging the tarp underneath will help to stop the ridge cord rubbing any proofing or coating off the tarp fabric.
3. Tie the ridge cord between 2 trees or whatever to rig the tarp - it'll be fine, just not taught along the ridge.
4. Get yourself 2 bits of cord to make the prussik loops - I'd guess 12 to 18 inches long will be ample. Use 2mm or 3mm cord for this depending on your ridge cord, the prussic loop has to be much thinner than the ridge.
5. Use some simple overhand knot to make each thin cord into a loop. Don't do this too tight because you're going to undo them again in a minute.
6. Fasten the thin loops around the ridge with a prussik knot. One at each end of the tarp, between the tarp and the tree.
7. Get the prussiks just right.
8. Then doing one end at a time, undo the simple knot that closes your prussik loop, pass one end through the tape loop at that end of the tarp's ridge and knot the loop closed properly this time.
9. As soon as you've done this, you can pull the prussik knots outward to tension the basha along the ridge.
The trick is, once you've set all this up, you don't undo it. You leave the ridge cord threaded through the loops on the tarp and the prussik loops tied around the ridge cord. Just unfasten from the trees and roll it all up. This makes pitching really quick.
If you want to get fancy, put a short loop of strong shockcord between one prussik loop and the end of the tarp, then once tightened, the elastic will take up any slack if the tarp gets wet and stretches.
If your tarp doesn't have tape loops stitched on and uses eyelets instead, you should tie very short loops into the eyes so the tarp can be slung under the ridge cord as described. The pussiks loops are set up the same way but tied directly into the eyelets instead.
If you don't already know it, learn the double fishermans knot. This is the best knot of all to join two rope ends together into a permanent loop. This is what I'd use to close the prussik loops permenantly once they''re threaded onto the tarp.
Lastly, this approach tackles hanging the tarp from the ridge and tightening it as two separate jobs. This is essentially the way hennessy approach it with their hammocks. You could use the prussik loops to suspend as well as tension the tarp but I like my way best. Have fun experimenting and you decide for yourself.
All the best,