I pull my backing real taut without overstretching and tape it down with painter's tape. (Usually, I work on large tabled pushed together at work to do this) Then I lay the batting down in halves and smooth it out on top, patting as I go. I repeat with the top and pin about hand-width apart.
Then I check the back and make sure all is smooth.
By using that method, I have had no puckers using the alking foot for SID or other straight stitch quilting.