Ever wonder how you can fit a corset when it laces up the back? Since a corset should have 2-4" of gap in the back for the lacing "spring" it can be difficult to bridge that gap while fitting. Some people add the gap back in and pin it shut, but that doesn't accurately reflect the flexible nature of lacing or show you where tensions properly lie. The best way is to lace up the mock up just like you would the final corset. Setting all those grommets is a waste of time and money, though. To make up for that make a pair of Lacing Strips to baste to your corset. They're quick to make, and you can re-use them over and over for corset fittings without wasting a ton of grommets.
As you can see lacing strips are really simple. Cut two strips of fabric 8-ish inches wide by 12-20 inches long. Fold them in half and sew a seam 3/8"-1/2" from the fold (I use the width of my presser foot from the needle in middle position and it works really well). Sew another seam 1 - 1 1/2" away from that, then sew another seam 3/8" -1/2" from that. Put bones in the 1/2" channels and finish all the raw edges (I serged them, you can zig-zag just as easily. I don't recommend pinking because you'll be picking these out a lot so the edge will ravel). Then set grommets every 1 1/2" in the gap between the bones.
That's it! You've made a pair of lacing strips! Now you can baste them to your pattern where the grommets go so you can lace up your mock-up just like your finished corset. If your lacing strips are too long just let the top/bottom hang off. I like to center mine, but it's really a matter of taste. The important part is to just lace the grommets where the corset is, so don't try to lace up any gap, just let it sit there so that it doesn't pull strangely on your mock-up.