I lay a king sized sheet on my living room floor and pin into the carpet using T-pins. I then pin the backing to the sheet, spray with basting spray then lay out the batting and straighten it out and then spray and put the top on the batting. I have had very good luck doing this and the spray does not get on anything but the outside edge of the sheet.
By pinning the backing to the sheet and making sure it is very taunt, I do not get any pleats,etc.in the quilt. Hope this makes sense!!!