Prism99

After you have measured and cut the border, you pin it to the quilt -- starting with the mid-points and ends. Halfway between each of those pins you pin again. You do this over and over until you have pins every 4 inches or so. What you may notice is that in some 4-inch sections there is more border than quilt, while in other sections there may be more quilt than border. "Easing in" the fullness means stitching the two sections so they match -- but without stretching the short one to match the big one. Basically you are manipulating the fabric so there are tiny, tiny little bubbles in the excess fabric with every stitch.

If the quilt looks larger than the border, you want to sew with the quilt side next to the feed dogs. The feed dogs move the fabric; the presser foot does not. This means that a little more of the bottom fabric will be stitched to the top fabric. Using a walking foot makes this process much more even because the walking foot works at feeding the top fabric; however, it's still a good idea to place the fuller piece next to the feed dogs.

In an extreme case where some sections of the quilt are too full while others are too scant, you might want to flip the quilt sandwich over so that the fuller piece is always next to the feed dogs. Also, in an extreme case where there is way more quilt than border, you can sew a gathering stitch along the quilt's edge and pull that up to make the quilt fit the border length better before you start sewing.

There are probably sewing sites that demo easing in fullness.