I have used Hobbs 80/20 fusible on a twin size and many smaller sizes. I love the stuff! I iron it on the old carpet in the basement. I found that trying to iron on the ironing lard was too difficult and I couldn't get it as smooth as I wanted. First I put on my knee pads from the $store. I lay out the backing on the old carpet with wrong side up. I smooth the batting with my hands on top of the backing. I then smooth out the top on top of the batt. I start in the middle of the quilt top and iron across it right and left. I press down to iron with a nice pressure before moving to the next spot. I then gradually iron across the next section right and left until I have ironed out to one end. I then start back in the middle of the top and iron the second half in the same manner.
I flip over the whole sandwich because the sandwich should be one piece now. I start the same press/iron method in the center of the back and iron outwards towards the edges. I find that by the time I get the backing ironed to the edge, I have to peal the edge back and re-iron because the fabric has advanced a little more backing towards the edge. Since the backing is what I can't see while FMQ I make sure it is perfect. I place a few safety pins along the edges in case I catch the sandwich while quilting and I don't want the edge to peel. I start FMQ in the center and work out. When I get the center third in the middle quilted, if needed I re-iron the back. I have only used the Hobbs so I can't say if the other brands are the same. Hope this gives you some ideas on how to proceed. Good luck.