I always sew on my borders with the quilt top on the bottom, the border piece on top. Even with a walking foot this helps "ease" in the quilt top and keeps the border flat. Clear as mud?
Be sure and measure 3 times, once thru the middle and both sides and then take the average. Some people just take the middle measurement but I like to average. Do the same procedure for the top and bottom borders.
Raggiemom, we were typing at the same time!