I spent an evening making practice sandwiches for the free-motion quilt-along and challenge. These are easy to make.
Flip it back over and smooth again, at least once more each side. I used a cotton batting, which sticks nicely to fabric. You may need to fasten your bottom layer down if you are using poly. (Blue painters tape works fine on tables and cutting boards and hard floors. Try pins for carpet.)
With the top-side up, baste with your favorite method. I tend to use pins. Spray baste is fine if you like using it, or thread-baste, or if the layers are staying together well and your piece isn’t too big, straight-stitch a few lines to hold it together.
And there you are. Practice quilt sandwiches. Now go practice.