A 3-way mirror is extraordinarily helpful on this mission. In fact, I'd say it's a must for all hair styling endeavours. If you don't have one, install one of those handy little mirrors on a flexi-arm. You really need a way to see what's up in those nether regions, it'll save alot of unnecessary frustration.
Probably the most common mistake people make is trying to work with the hair all in one section. With medium hair cuts, it's especially important to work in sections, both to properly secure the style and to maximize the length.
And wouldn't you guess, the next step is to part off the top section (from the temples back) and twist it upwards. Use your index fingers, bring them together in the back, grasp the section in one hand and use the other hand to separate the section from the hairs that drag in from underneath. Keep it clean. Hold the section in place and look at the shape of the hair in the crown and sides. Is it how you want it? Is it too flat? Are there some snags that need smoothing? Now is the time to take care of all that.
Pinning must be done properly to secure the section. I've colored the tip of my bobby pin in this photo to demonstrate that one half of the pin goes into the twisted hair and the other half goes into the hair laying against the scalp. I almost always use two bobby pins to anchor the section well. Angle the second pin slightly so it'll cross over the first, but still remain hidden.
Twist up and secure the last section. If your hair isn't quite long enough to reach the center without coming loose, (common with medium hair cuts) split the section in half at the middle/back of your head and twist each section separately. (which was the case with this style)