When I used to pin my sandwich, I fastened my backing down to my table using bull dog clamps on the table edge or on the floor with masking tape. If you slip a marble under the backing, this will give you a little space to get the pin under the backing and back up through the layers. The marble can be rolled along as needed to the next pin spot.
Make sure your backing is about 2 inches bigger all around the size of the top. Lay your batting on top of the backing (cut the same size as backing) and smooth the top over the batting. You are now ready to put pins in about every 4 inches apart.
Start quilting from the middle of the sandwich outwards so that any wrinkles can be moved to the edges. Check the instructions on the batt package for how far apart the quilting lines can be. Good luck.