Hanging Leg raises are a staple in my abdominal training. I've been doing them for years, but not always with the best form. The most common mistake is bending the arms and leaning back as you lift your legs, which turns the movement into more of a lat exercise; That's not necessarily a bad thing, but it wont be as effective at strengthening the abs if that is your main goal. That being said, strict leg raises will strengthen the lats, triceps, and quads in addition to the abs.
In this video I demonstrate a few different leg raise variations, moving from more to less difficult. I've always struggled with endurance with these types of movements, so by the end of this short video I'm actually getting fairly tired! If you have access to a set of stall bars (ladder bars), then these will be significantly harder to perform as they don't allow you to swing or lean back as you lift your legs. Sometimes I improvise a set of stall bars by placing a barbell inside a power rack and holding on to the pullup bar directly above it, so that the bar is behind my mid back. Both versions have their benefits.
If you really struggle with raising the legs past a certain height, then you might need to spend some time working on the flexibility of your posterior chain (calves, hamstrings, spinal erectors) and possibly even the lats as well. It will be very tough to get stronger if your own restrictions are holding you back. The more I work on my flexibility, the easier these movements become. Of course if you are just very weak in the abdominal area, then these hanging leg raise movements might be too advanced, and starting off with more basic exercises (planks, hollow body hold, reverse crunches) would probably be more productive initially.
With hanging leg raises, it is much more important to focus on keeping strict technique than it is to perform a lot of reps. Give these a shot, and if you have any questions, hit me in the comments below!