When I get ready to do my sandwich, I use the big tables at church. I tape down the back, using masking tape at about every 7 inches. I adjust it to make sure it is totally flat. If I mess this up my quilt will be the same. I have tried doing this at home but it does not come out as well. Once I had to take all the pins out and start over because I was not happy with the result.
I do hope this helps you. I know how it feels to rip out more than once.