Even as a lesbian with a long standing sexual relationship, even I find it hard to write yuri sex scenes (though, strangely enough, yaoi comes quite naturally...), but I will do my best to give any advice that I've found helpful.
As a lot of the others have been saying, considering the background of your characters is very important. If you have a bold character, a shy character, a virgin character, an experienced character, it affects how the sex will develop overall. It's best to go a little in-depth when you are describing them to make them more of a person rather than a creature about to have crazy-hot sex.
For me, emphasizing the softness of girls comes to importance, not in manner but in their bodies. Girls are a lot of soft, curving planes no matter what build, though it can be emphasized differently if you're looking for a rough, dominating sex scene or something fluffy and loving. That, and the realness of it is important, too. With sex there is usually a lot of fumbling, getting caught in clothes and strange noises and new touches. I think it's better to describe those little things as well, that and making sure not to overkill on dialogue. Though some is good during sex if well written, there gets to a point where the pleasure should be well enough to keep them from talking, or at least from talking completely coherently…
When it comes to the mechanics itself, I think that that's where yuri becomes more difficult for me. There aren't a lot of words that sound as pleasant when describing female anatomy, it seems, without being either slander or almost medical. I've heard things ranging from pussy, vagina, juncture, mound, lips, sweetness….It's really depending on the mood of the piece. For me, that is honestly where I struggle the most-- the intimacy and foreplay comes easily, but the words to use are difficult (compared to with men where it's almost a glorification-- member, shaft, cock…).
Also, I'd say with female sex it becomes a lot about pleasing the partner rather than the self. Unless they are touching each other at the same time (which becomes difficult if you are trying to do things to your partner and concentrate, or unless sixty-nining…) generally (or at least in my experience), it's about taking turns. One partner usually makes sure to get the other off first, then the roles are switched. That also goes into the personality of your character-- whether they would please their partner first, expect to be pleased, and the way in which they please each other. That combined with the fact that girls can potentially have multiple orgasms is enough to create quite a lengthy sex scene.
If you're going for something a little less graphic, even just the slighest bit of affection can start everything. My first real kiss with a girl (and the same girl I'm with today) started when we had been rolling around in her bed, sticking out our tongues at each other playfully, and it evolved into something much more. It can be from something playful or even something angry that a relationship starts. It's not too much unlike yaoi or het pairings, really, how it all begins, though with girls I do want to say that there usually is a little more emotion involved from the get-go, if only from a prexisting relationship or the societal deviancy of the action. It's best to explore those emotions ranging from confusion, acceptance, and taboo, depending on the situation.
I have a problem with a lot of yuri I see written-- it's usually very cliché. The most important thing I can really say is to make it real; just because it's two girls doesn't make it perfect and beautiful and sexy. I think it's best written as if you're peaking into someone's real life (though maybe a little glorified-- it is fan fiction, after all) and getting a peak of intimacy from the inside.
Hopefully something I said helps and isn't just a late night/early morning rambling :3