First, I lay my quilt back (right side down, of course) flat and use masking tape to adhere it to the floor so it stays as flat and taught as possible. I use 1.5" or 2" wide masking tape (the wider the better) and I tape the quilt back down with newspaper to protect my hardwood floors. I leave a small gap between the quilt top and the newspaper so the tape can also adhere to the floor, securing the quilt top. The first time I spray basted in the house I didn't use newspaper and it took me about an hour and a lot of elbow grease to get the dried glue off my floors. I learned that lesson quickly.
Then, I lay out the batting over the quilt top, smoothing out any creases. Once I am happy with the positioning I fold up one short side about 12" or so and I spray the basting glue onto the quilt top all he way across and then I fold the batting back down, smoothing out all the creases and bumps.
After I've secured that end of the batting to the quilt back I move to the opposite side of the batting and I roll the batting all the way back to the other end, stopping where I can feel the adhesive on the other end doing it's job. I will be on my hands and knees (no shoes but with clean feet!) In the center of the batting that's been secured down and continue my way down through the middle of the quilt as I tack the rest.
From here I start spraying the adhesive about 12" wide from one side of the quilt to the other. Depending on the brand of spray baste you're using will dictate how densely you apply the glue. I usually spray from one side to the other in a zig-zag motion, and then sometimes add one more spray, directly across in the opposite direction for good measure. (I hope that makes sense!) Then roll out the batting, to around the point where you think the glue ends and smooth out any bubbles and creases. Repeat this until the entire batting is tacked down to the quilt back.
After the batting and the quilt back are adhered together I repeat the entire process to secure my quilt top to the batting. Once it's all sandwiched, I remove the tape and it's ready to quilt. I am sure there are lots of other ways to spray baste out there, this is just the method I've developed that works for me. Do you do it differently? What works for you?