I bought a can of 505 basting spray and do not know how to use it. Any help appreciated.

Place and smooth out your backing on the batting, then fold half of it back. Spray on the batting then smooth out your fabric back over it; doing the same thing again with the other half. Then turn over and do it with the top fabric. Just a light even spray is all you need....if you have wrinkles or bumps it is very easy to just pull back the fabric and smooth out again. 505 is great stuff...doesn't gunk up the needle and doesn't have a bad odor. If it is a good sized quilt, you should still do some pinning; but it doesn't have to be close like you would without the spray.
Good luck.

If you are working on the floor, I would spread out a large sheet first and tape to hard flooring wth blue painter's tape (or pin to carpeting with large straight pins). The sheet will catch any overspray.

Smooth out your backing, wrong side up, on the floor. Your batting should always be bigger than your top -- 2 to 3 inches bigger on each side for a lap quilt or smaller, 3 to 4 inches for a larger quilt. Batting should also be cut bigger than the quilt top, about the same size as the backing.

Lay the batting on top of the backing, fold back half of it, spray that half of the batting, then fold it back and smooth it out on top of the backing. Once you are satisfied with the positioning, fold back the unsprayed batting, spray that, fold it back and smooth it out over the rest of the backing.

Once you are satisfied with the backing/batting, smooth the quilt top over the batting. When done smoothing, fold back the top half, spray the batting, thn smooth the top half on top of the batting. Do the same with the bottom half of the quilt.

If it's a large quilt, at this point you may want to add safety pins along the edge about every 6 inches. Smaller quilts usually don't need this.

The spray allows re-positioning for at least half an hour, so you can lift and move things around until you are satisfied. After it dries, the quilt sandwich should be very stable for machine quilting.

Spray basting is used primarily for quilts that are to be machine quilted on a regular machine (not a longarm).