For me, the easiest way to machine quilt is to use a walking foot and make gently curving lines. A lot of people use a walking foot to stitch-in-the-ditch, but I find it makes me too anxious because it's hard to make it perfect enough to satisfy me! With curving lines, nobody knows if I'm a little off.
Free motion machine quilting takes some practice for most people. It's good to practice on utility quilts first (baby quilts are very good for this because babies don't care how perfect the quilting is!). It's important to spend this kind of practice time before trying to do it on a large important quilt.
In my opinion, it's well worth the investment to purchase a walking foot for your machine so you can machine quilt at least some of your quilts. It helps get them done, finished, and out of the way so you can start new quilts. :-D Free motion quilting is very popular, but it's not the only way to machine quilt!