It's a bit like being the plunger in a huge syringe.
It's totally and completely painless, but a bit noisy . It is, however, quite light inside, so although there is nothing to see, it isn't dark.
They gave me headphones to cut down the noise.
I tried counting to make the time pass which worked for me. I'd advise thinking of something like that , that you can think of whilst you're in there, otherwise what is quite a short time really (a few minutes) seems endless.
Hope all goes well with you. It did for me.