Lightly, and spray from at least 10" from fabric surface, NEVER near your machine (the glue is a vapor that could slip into the interior of your sewing machine and gum up the works), in a ventilated area. Through trial and error, you will discover exactly what lightly means, since as you correctly pointed out, we tend to think we haven't sprayed enough and the spray won't make the batting and fabrics stick together.
It is not good to have over -spray wind up on your floors, as it can be difficult to clean up. For that reason, I spray out doors on my patio on a sunny, windless day.
When the quilt is completed, I wash it to get the basting glue out of the quilt.
Have fun!