I'm speaking from personal experience here - this is about egodystonic sexual inhibitions (inhibitions you don't want but still have) and of course you need to mentally prepare first to get rid of it. It needs to be intentional.
Examples are:
- Problems with having an orgasm during sex (especially as a male, I don't know if this works for women)
- Problems talking to your partner about a kink or something similar, where you WANT to talk about it but CAN'T
- Problems with opening up emotionally in general to intimacy
So, it works like this:
First of all, go into a mindset that this is not about having sex afterwards. This may happen, but this is not the goal.
Get undressed (both), partner should use an oil for the massage which a) is warm and b) has an enjoyable smell.
Lay on your belly, partner should go for shoulders and back, also the arms if you enjoy that (I do, not everyone does), don't go below the waist for this. Just relax and enjoy the sensation. If your partner is not too heavy they can sit on your butt or similar.
Let the massage be slow, gentle, non-sexual, but intimate. Accept if you fall asleep, but also accept if your thoughts start to run wild, this is a good sign.
The active partner should start talking about intimacy and positive things in a soft and soothing tone, encouraging. But don't touch the actual topic, let the sub-consciousness do the work here. We're talking about intimacy here, not about achieving something.
Take your time.
After that, when you're fully relaxed, make out, the both of you. :-) Just make out. But take your time until you get there. Relaxation and intimacy is the most important thing.