It's actually more important to stabilize the background fabric first. When applying a large applique, the largest risk is that the background fabric may become distorted as you work around and you end up with wrinkles in the background fabric. My advice is to heavily starch the background fabric before starting; a 1:1 solution of Sta-Flo and water will work. The fastest way to do this is to "paint" the solution onto the fabric using a large wall painting brush, wait a minute or two to allow the starch to be fully absorbed into the fabric, toss fabric in dryer, then iron with steam. This will stiffen the fabric and keep it from distorting while you machine sew around the applique. If you don't use starch, then you *really* want to use some kind of stabilizer on the background fabric. This is much more important than stabilizing the applique itself, although you can do that too.