Prostate massage noobie looking for guidance

Hey everyone. So a little background: I am no stranger to anal play (I own some dildos, plugs, etc) and have bottomed numerous times. However, I am new to this prostate massage thing.

I found out about the Aneros on some reddit thread about male sex toys, and have been pretty intrigued after finding out about it.

I am wondering about what model to buy, first and foremost, and any additional things to buy along with it.
I do like the feeling of girth and fullness, but it seems as if a midsized model would work best for a beginner. I am a bit split between the Helix trident and the MGX trident.

Another thing I’m wondering about is what to expect my first few sessions, and how to go about it. Not sure if I’d lay on my belly, back, or what.

Any advice, guidance, or anything would be super helpful. Thank you!