Step 1: What you need
-one package satin blanket binding (at least 140 inches long, I use Wrights)
-a sewing machine and knowledge of how to use it
-something to mark your fabric like tailors chalk
Step 2: Wash your fabric
Step 3: Measure your fabric
Once you have your marks, pin the pieces together near the marks so that your fabric doesn't slip when you are sewing.
Some people put little yarn ties through both layers at measured intervals or use another quilting technique so that the fabric doesn't slip against itself when it is a blanket. If you want this, then now is the time. In my experience, these blankets are small enough and flannel sticks to itself well enough that you don't really need it for practical reasons.