Say hello. Frankly, half the time it never gets past that. Some people just radiate GTFO as soon as you (or I) become part of their universe.

Ask her about herself such as how she knows the host or whether she likes the beer or whatever. Make it something not too personal but revealing of personal taste or social connectedness.

As soon as anything you have in common is revealed, talk about it. E.g. talk about the host in a non-gossip way. Talk about the beer. Talk about the music.

Close the gap: Ask her to sit and continue the conversation away from noise or interruption or ask her to dance depending on the type of party.

If someone you know walks by, introduce her. If she already knows them, it's another point in common.

Most often, that's all that will happen. She will get pulled away or walk away or the party will end. You may never see each other again, or your social universes might start mixing and you do meet again. If so, she will remember the pleasant conversation and may want to repeat it. In my own case, I tend to start very curious and genuinely interested in a new person. Many times, I lose interest. It isn't anybody's fault. We might have conflicting personalities or nothing in common or bad chemistry or whatever. Usually, both people will detect that the conversation is going nowhere and politely move along to meet other people.

In very rare cases, you both fall in love, get engaged within 3 months, and have three children together. That's what happened in my case except that I didn't meet her at a party. I met her in a noisy dining hall. Almost all of the above steps work in dining halls too.