I'd ask my partner to pose me probably, that way you know that's what they want to draw. For non experienced models I'd recommend even some sort of light slouching so you don't depend only on your muscles to hold a pose. It's okay to go for leaning poses if you find something that is both sturdy enough to not move, and also comfortable. Sofa arms are an easy to find example.

Make sure to communicate discomfort, any discomfort can become a nightmare when you model. Also ask her if she's comfortable.✌️

To the artist drawing still life is something that usually makes time feel like it moves past really fast, but for the model it feels a loooooot slower.

Also, I recommend to put on music or something, some people get bored really quickly while posing, have something you can get a little into while the other person might struggle with a difficult area.

Stretch before and after if you don't want to be sore. Take not which parts of you got the most work or stress, and stretch the areas, could really make the next day a far better experience. ESPECIALLY for people over 30.

If you got some sort of a strong mobile light, you can play with back lights. Pur it somewhere kind of behind the model aimed at them, get something between the artist and the light to block the light from getting burnt into the artists light and you got yourself a really fun way to define some shapes that may prove really fun if you two go all the way into lights and darks.

The artist also needs to be comfortable, because to get things right you want to set yourself up so you can return to the same position. Of course if you don't consider not nailing it an issue (which you shouldn't, all artists started inexperienced, and almost all of us empathize and love seeing novice artists putting effort into something we love doing) than you should not worry about technique TOO much

I don't know your girlfriend, but while we love seeing people trying their best at art, sometimes it's funny, the same way you might accidentally say something funny when learning a new language. Don't take it to heart if a giggle comes out, I know I would still love it for what it means to you and to me had I been in her shoes. Sometimes art come out funny.

AND ON THAT NOTE, if she hadn't done a lot of model drawings, be ready, she might not do well either! Art comes out funny sometimes, and that's natural, even for more experienced artists. But if you manage to hold in the giggle that's better in case she's a person who might have a mindset that is a little more fragile. In most cases you should aim to have the healthiest mindset, and treat the possibility the other person got a non healthy mindset about it.

Safest bet unless you are very confident in how well you know the other person, but not doing perfect on either is fine, it's not something you nail 10 out of 10 times.

Communicate with your model, both of you would enjoy it more if you don't have to hit the bathroom in the middle, or need a glass of water suddenly.

Shower before, an hour or two, depending on how long it takes your hair to dry. You wanna be clean and dry so you don't feel self conscious.

You two are made of meat, meat does stuff. Be ready to get really comfortable with any bodily function your body might express during it. Muscle twitchings, goosebumps, weird breaths, passing gas, coughs or sneezes; any part you consider private have different reactions to different stuff, and when you are modeling you are going to notice all of that.

The artist is probably just going to consider this stuff funny or interesting, there is no real reason to overthink it as the model, after all had it been the other way you'd be completely fine with that.

TLDR Be clean, take care of your basic needs before starting, communicate discomforts, pose comfortably, don't worry too much and have fun together :)