In sex between women, things that are considered "foreplay" or pre-sex playing around in traditional male-female sex become sex itself. So oral sex and digital sex (using the fingers/hands) are sex. Foreplay also takes centre stage and can last a long time. Actually, I don't even think of so-called "foreplay" as foreplay, since it's such an integral part of sex...
Foreplay can include gently kissing/sucking the neck, caressing the breasts, and teasing the nipples with your fingers or with your mouth.
A lot of people emphasise that you should touch the breasts gently and slide your hands along the skin in order to caress them, and never to knead them like bread, but... I personally think a little squeezing is nice on a psychological level, if not on a physical level, so long as it's not hard enough to cause pain.
You should treat nipples gently to begin with. They can take harder sucking and pinching as you get further into sex and the woman becomes more aroused, but it will just feel unpleasant if you go straight to nibbling them.
Sometimes sucking more harshly on the neck can be nice, but it's also painful. Creating a lovebite always comes with a bit of pain.
There are less conventional forms of foreplay that are important, though, and should really be done before you jump to the above mentioned areas. Kissing, for example, is the best first element of sex and should go on as the only element for as long as it takes for both women to want to go further. (Ideally. Women also get impatient.)
The human body has a surprising number of places that are nice to brush your fingers across or slide your hand along gently. The back of the neck, the back, the shoulders, the inner elbows, the thighs, and the back of the knees are some such places - although the elbows and knees are a rare place to target, maybe because they're not well known.
That's why just having someone slide their hands down you when you kiss can feel pleasurable. Not on the level of your nipples and such, but it's a great place to start to get a woman aroused.
I can't say that having your butt grabbed is physically pleasurable, but it's nice on a psychological level. It's a sign that you're being desired, you know?
When it comes to oral/digital sex...
The entirety of the external female sex organs are sensitive and pleasurable to touch, but the clitoris is the star of the show. It's often described as a "little bean" up at the top, but it can vary a lot in size.
Here's a diagram:
https://myhealth.alberta.ca/Health/_layouts/15/healthwise/media/medical/hw/h9991308-001.jpg
The clitoris is super sensitive and really the best way to bring a woman to orgasm, if she has one, but you need to be careful not to overstimulate it. Some clits are so sensitive that they can't be touched without the hood (a sort of foreskin) over the top or else it will be shockingly painful. Too much vibration from some sex toys can also overstimulate the clitoris.
Once its overstimulated, the clitoris won't feel pleasurable to touch at all for a period of time... So unless you're prepared to give up and cuddle for a while and try again later, an overstimulated clitoris spells the end of sex (or at least no orgasm for that woman).
If you're using your fingers, they should be clean, and your nails should be short and not have any nicks or rough edges on them. One or two fingers is really enough for touching the genitals.
You'll also have to use a little lube, or you can try and use the secretions your partner has put out because they got wet, but you'll have to actually transfer that from the space in between the inner lips/before the actual vagina entrance (which isn't well represented by the diagram I linked?). You don't want to touch the genitals dry, because it won't do much.
Some women also don't produce a lot of secretion no matter what, so always have lube handy. Make sure it's water-based and that it's real lube: make-shift lubricants like oil or vaseline do not belong on the genitals, as they can disrupt the balance within the vagina if they get in and cause an infection. For the same reason, don't use flavoured lube.
Also, I've noticed that there is such a thing as too much lube for the clitoris - at some point you won't be causing friction anymore, and there's no pleasure. Use just a dab and add more if it feels too dry. If you add too much, you can try gently dabbing the excess off with a tissue.
If you're using your mouth, be careful not to bite or catch her with your teeth. Sometimes it's helpful to cover your teeth with your lips.
I wanted to link to the guide to cunnilingus (oral sex performed on a woman) that I used a long time ago, but I can't seem to find it...
Well, whether with your hands or your finger, it's pleasurable to gently rub the inner and outer labia, the space in between them, and that space just before the entrance to the vagina that I mentioned before. You can really take your time - pleasure is better than an orgasm, and the more pleasure you give beforehand, the better the orgasm will be.
When it comes to the clitoris, you can gently brush up and down across it, draw circles over the top of it, suck it a little, or even tap it repeatedly. Keeping a moderate pace will bring a woman to orgasm (except maybe with sucking; I'm not sure if that alone can do it) - you don't need to speed up. In fact, too much speed causes no pleasure.
The best sex won't go to one and just keep going to push her to finish, but will switch the patterns and pressure to keep it new and pleasurable and draw it out. Some people even suggest moving away from the clit again and teasing the labia, or even the rest of her body, before going back.
Not everyone enjoys penetration, and not everyone can take it. But for basic vaginal penetration with the fingers, you need lube and usually two fingers. You're supposed to find the rough patch against the front wall of the vagina, closest to her clitoris, and make a "come hither" motion against it.
Note: For the vagina, there really is no such thing as too much lube. Natural secretions will eventually dry out with use and it will become painful for her when you move.
How much pleasure a woman will feel from penetration varies. Some women are really sensitive and get a lot out of it, and for some women it's just an annoying feeling. Only about 80% of people with vaginas can orgasm from penetration.
When it comes to scissoring... It's not popular, but I've heard there are some people who enjoy it. Generally it's hard to rub your genitals against someone else's like that and it doesn't end up being pleasurable. Scissoring also puts the rest of your partner's body a very long way away from you, so there's nothing else you can really do.
Some women apparently rub their genitals against their partner's thigh, and then their partner can somehow rub against their thigh at the same time too, and they orgasm that way. How? Sorry, I don't know: thighs aren't hard enough to be pleasurable to rub against, in my opinion.
Oral and digital is easy because it doesn't need any extra tools like pegging and toy use, so I consider that to be "basic sex". Other people might disagree, though.
Oh - and don't put any perfume or "feminine hygiene" products/washes on the genitals, either. I've seen some yuri where they perfumed the clit before sex... I can only imagine the burning pain and infection that would really result from that.