That may take some practice. I did it myself once, and was off by about 4 mm. The resulting glasses were incredibly painful. My eyes would try to look in two slightly different directions, and my brain would put the wrong 3D picture together. Whenever I moved my head my eyes would readjust, which took a noticeable period of time, and I could literally feel my non-dominant eye turning itself to find the focus. I chucked the glasses after a few days.

I'm sure it could be done better, but boy, that was an interesting experience I hope to never repeat.