Trim seam, clip curves

My experience is that before doing an understitch or turning a seam inside-out, there is usually a step that says "Trim seam" and if the seam isn't on a straight bit of fabric, it's followed by "Clip curves." This can be across any number of layers of fabric and/or interfacing. All this means is that you are going to trim down the seam allowance from a previous seam, and then make notches in the curves of the allowance. This will help ease folding the fabric over to prevent wrinkles or bunching.

When you trim the seam, you basically want to trim it to where there is still a little bit of allowance for you to do your understitch through, so don't take it right to the seam, give yourself about ¼"-⅜" of an allowance, depending on how comfortable you are with your sewing machine--smaller is better. Even if you're not doing an understitch and you're simply turning the seam inside-out, you don't want to clip right up to your seam simply because it will increase the chance of the seam coming undone.

Furthermore, you want to cut the seam on a bit of a bias to help ease the fold. I usually tilt the scissors away from the seam while I'm cutting, about a 45° angle to help ease the fold under in the later steps.

After you trim the seam, you want to clip the curves. This means that you create a series of notches around the curved edge of the seam allowance. Do not break the stitch that you did, however. The tighter the curve, the more clips you want to make and the closer together you want them to be, but generally I find every inch or so is fine for most of the notches.

After you do this, there is usually an ironing step before you stitch. When you iron, pay attention to how the outside of the garment will look. If you can roll the exterior seam to the inside by just a hair, it will usually result in a more attractive garment.

Finally, you can pin-baste (if possible) and understitch as you would.

For more on finishing raw edges, see Finishing a raw edge with a facing piece and Finishing a raw edge with single-fold bias tape. Both of these instructions involve trimming seams and clipping curves.