Well, it could feel like a number of things. You specified "feel," so I won't really describe common hallucinations.
But, usually it feels like you're sinking into your bed. A vast amount of weight on your chest or perhaps a slight feeling of hypnic jerk.

Now, if you enter Sleep Paralysis in a state of fear, you run the risk of it manifesting itself. Due to accompanying hallucinations, this would be a very undesirable effect.
An ominous feeling of dread. Likely with the feeling of being watched.

The important thing is to not enter sleep paralysis fearful. Sleep Paralysis are proven to be completely harmless and a very common.
Most people enter in a state of fear because they are unaware of what is happening and that feeling is manifesting itself into terrifying hallucinations.
You come in with a huge advantage. You know exactly what is happening and that it is natural. So, there is no reason to enter fearful.

Sleep Paralysis feelings are large gateways to lucid dreams with definitive indicators of exactly where you are in the process. Welcome them and be thankful when they come.
Enjoy your journey.