I had the same problem during my first attempts.
Since I didn't have a blindfold, I made my room completely dark at night.
This way, even if my eyes open by themselves, I could just ignore it, because I didn't see anything that would distract me, and they would close again after a few seconds.

It is normal that your eyes will start moving around rapidly, it's weird at first, but if you don't focus on it you'll stop noticing that it happens.
Sometimes you may also feel strong vibrations in your eyes / eyelids. Again, just ignore it, it'll go away.