|To roll a facing seam, first press the seam open then closed, rolling the seam slightly to the facing side. After one applies a shaped facing to a front edge or neckline, there is a seam right on the edge. The edge will look more attractive if this seam is hidden. One can do this by rolling the edge.
First press the seam open then closed, rolling the seam slightly to the facing side. To clarify, pinch the edge with your thumb on the facing and your first finger on the garment (like you're signing "OK"). Then you slide your index finger a little bit down your thumb towards the base of your thumb. This results in the facing seam being just slightly hidden by the garment.
You press the seam into this position, but that's often not enough to keep it that way--so you have to stitch it into position. One way is the pick stitch, a very small hand back stitch that can be done from the facing side or garment side, depending on the look you want. Another way is machine topstitching. One could maybe even fuse it. Whichever way you chose to do it, the result is the same.
If I've been clear enough in my description so that you understand what's happening , the technique itself is pretty easy.
If you're topstitching by machine, first you temporarily secure the rolled edge. Basting glue, steam a seam, hand basting, or pin basting can be used. After the facing edge is basted, you should not be able to see the seam from the RS of the garment--it should look like a folded edge instead of a seam. Then you topstitch.
If you're stitching by hand it's easier because you don't have to baste. Instead you just pinch the seam into position with your left hand while you stitch with the needle in your right hand. This gives you better control and is more inconspicuous, but takes a lot longer.
I've tried my best to explain this without pictures. If there's anything that you cannot understand, please comment and I will try to clarify.