I tape the backing down very well. Ie start in the opposite middles and pull taut but not tight to stretch fabric. \\\\\\\\I usually end up with a good tape to stop paint going on walls(forgot name) every 6inches. opposite middles the corners square up and can take out creases as going(hope it is ironed) batting on tease into place with finger tips only no pulling as it will stretch. Now carfully with help if possible place top on. Again smooth out with finger tips no pressure. Pin using safety pins as close as possible with as much material in as you can. More pins stabler the top .
PS keep watching tape has not come undone. polished surfaces not ideal.
Best of lick following my ramble.