After pinning carefully by matching shoulder seams and centers (I make a little snip in the center of each facing-- before you unpin anything from the pattern, mark all the centers with a small snip or mark in the seam allowance! It saves a lot of frustration later), sew slowly around the neckline facing.
The instructions say next to "clip and grade" that seam allowance. I find it easier to do that the other way 'round. First I "grade" the seam allowance. That means cut each of those seam allowances slightly layered from each other. This is to reduce bulk.
The seams you graded should be under the facing, and the rest of the garment should be on the other side (my garment has a collar, and that went on the other side, too). In the above photo you see the facing (with the seams underneath) on the right, and the garment (with my collar) on the left.
Stitch very close to the garment, but stay on the facing side, and catch and stitch that graded seam allowance to the facing side. You need to put one hand underneath your sewing as you go to make sure the seam allowance is staying there with the facing.
Next, my friend's pattern says to tack the facing to the shoulder seams. On her style of dress, that is sufficient, but on some garments I like to keep that facing really down by stitching all of it down, or as much as I can get to.
On my dress, I wanted the facing down all the way around. I use a simple stitch for this-- just pick up a little bit of the garment on the needle, then a little bit of the facing edge, and pull the thread through. Make the next stitch about a finger-width apart from the last. You will want to use a thread that matches your dress material for this.