If it is a baby quilt, I lay the 3 layers on a flat surface, throw in a few pins so I can carry it to the frame, lay the sandwich centered on the 2X4 foot frame and put the clips on 2 sides, remove the pins from the quilt, I turn the clips until it looks taut, put on the 2 other sides clip and turn them outward until the quilt is taut. I check under the frame to make sure the back is as taut as the top, if not I remove a clip and pull the backing and re clip. Once it looks smooth and taut on front and back, I press down in the center of the frame to give me a little slack for hand quilting.
If you are just tying you could probably just leave it taut.
This is how I do it for fluffy polyester batts but thinner batts you could assemble back, batt and top on the frame if it was just a baby quilt. Lay out the back and put on clips, smooth out the batt and re clip one at a time, smooth out the top and re clip.