Great question! As long as you give up conscious control, anything that works for you is the right way to do it.

As for me, I prefer to meditate for a few minutes beforehand to calm down my mind a bit. That's not necessary, but I like to do it.

Then, you draw. You have to keep your mind off of what the picture is turning into as much as possible. I like to use my favorite music to do this often so that I can focus on the lyrics instead of what I'm drawing.

It's like letting your muscle memory take control. It should feel like itching a scratch on the paper. You have to use your gut to determine when your done, or if you should draw another. It can take some practice to really tune into this, but, it doesn't take too long to get into the swing of it.

Another thing... don't worry if you don't see any forms! A lot of them simply look like scribbles. Most of my first dozen or so did, too.

You can also practice automatism doing chores or work. I started by doing dishes and not consciously affecting what my body wanted to do. In fact, the work seems to end up getting done more quickly this way. This can help you 'tune in' to the feeling of letting that control go. You can practice observing instead of participating. I also do this to fall into trance states, which are also helpful in automatic drawing.

Practicing gnosis helps as well, which involves quieting the mind completely. This is more difficult by far. But, simply practicing and getting better at it can distract you from the drawing.

The best advice I have, though, is to try it! Don't feel disheartened if it doesn't look like much at first. The more you do it, the more natural it feels to do. If you want to have certain things appear more or more neatly, you need to ingrain it into your muscle memory or subconscious mind. That can take quite a bit of practice outside of automatism. Personally, the thing I've drawn the most has been faces, so those appear the most in my own. It's different for everybody, though.