It sounds to me like you're trying to get this directly from the plank, and I could see that being particularly hard.
The main thing I would have to say is that you really need to drop your hips similar to the way you would in a CKR, and curl your spine into a C, hollowing your belly similar to a split-grip V lift (or spin), so that when you reach around underneath your bum, you have just that extra little bit of room. You're not trying to go from shoulder right to your spine! If you need to put your arm directly under one buttock, then do so. It's all personal.
The other thing is about your hands. I found very much that I would slide, and I couldn't get a strong grip with my hands in this position. I felt very unstable. So, first of all, you want to make sure you've got your thumb pointing up. People have said "Think of it like sticking your thumb up your butt," and I suppose that's true! The other really helpful hint I had was to switch hands. It seems counter-intuitive, but it will help in trying to pull a little bit with the top hand, so that your weight is evenly spread, rather than putting all of it on your wrist (which would hurt!)
Sometimes it helps to study some of the pictures of people doing wrist sits to figure out what their body is doing; I had to do that a LONG time before getting this move. I noticed that some people tend to wrist sit with their hips a little off-center, and some people really like to have it all aligned. Again, it's a lot about practice.
Give it time, because this move can be really scary for some!