I work on top of my bed, Caley, a bit easier for me. I cover it with a couple layers of brown kraft paper (I have a roll of it, reuse until it gets flimsy or otherwise needs replacing), that give enough of a surface that I can skate my pins off the top of it, or you can carefully put your cutting mat in the middle to start with.
Most of us start from the middle, that is you layout the backing right side down, and then loosely position the batting. Fold the batting in half, and secure the bottom layer -- canned goods work on the floor. Blue tape is one of my quilting essentials! Gravity can help if you do it on a table. Big old safety pins stuck into a foam mattress also work, but I don't ever do any actual work on my water bed. Working in about 1 foot wide sections, put down your glue or spray, pull the batting over, pat down, fold back, work forward. Then do the same for the reverse. While things are still damp/loose turn over and make any corrections you may need to make.
When using pins, you typically don't have the extra ability to work those first two layers together. Again, spread out that bottom layer and secure it. Spread out the batting, I use my 6x24 ruler as a wiper blade. Then the top, as smooth as you can get, tugging on that bottom to make sure it is behaving. Pin, goal is to yes, give yourself space to work but about ever 4-5 inches.
Now, take a look at the back and hope for the best. Is amazing the worst that can happen though... Decide on what you can/need to fix.
Edit/PS: I air fluff my poly battings just a bit before I lay them out to get rid of any packing creases. Only a couple minutes, only on fluff -- no heat. Bonded cotton battings I'll usually throw in the dryer with a damp towel on a tiny amount of heat.