There is an HB9 in my economics class that I plan to open. The class is in a big auditorium so I can't necessarily go sit beside her. I was thinking of trying to open outside before the class and then sitting beside her as we go in. What kind of opener should I use and how should I use it?

Sit next to her at the start of class.

You: Hey, I think I've seen you around before, what do you think of this class so far?
Her: blah blah (doesn't matter)
You: I dunno, i mean econ is interesting but it puts me to sleep. Sorry in advance if I pass out on your shoulder

Another possibility. Can either be sitting, before or after class, doesn't matter.

You: A friend of mine had this teacher last semester, she said ... blah blah <make something up to make her laugh>
Her: <She'll laugh or not, doesn't matter>
You: I know I'm funny but you don't need to laugh that hard. Damn!

Catch her after class.

You: Hey, did you just get out of <class name>?
Her: Yeah, why?
You: What do you think of it so far?
Her: <doesn't matter>
You: Yeah, it has it's ups and downs. Like, just now for example, I sat in front of this guy who was coughing and sneezing the whole time. That was definitely not cool!

What you say really doesn't matter, you just need to engage in conversation, seem fairly uninterested, and seem like you have something more important to do than talking to her.

Alright I like that. The reason I can't really sit beside her necessarily is because the class fills up within 5 more minutes and when i walk in I usually end up having to sitting in the boonies. Do you think I should talk to her over a more extended period of time or be direct about it?

Slower is always better in a class setting, because a) you know you'll see her again, why rush things and potentially send a bad message b) She isn't necessarily out to meet guy, as opposed to if you met her in a club, so logistically pulling her is more difficult, and c) being direct in this setting is weird.

I would say get her number as quickly as possible and try to set something up outside of class asap. This can be a study date before your next exam, invite her to a party you're going to, or agree to meet at the bars/clubs that people go to. Whichever you feel more comfortable with. I do want to emphasize the ASAP part, because you don't want to be: "That cute boy in my econ in class." You want to be: "That cute boy I just met in econ class." The former is just a step short of ljbf, whereas the latter is a potential mate.

So I just read what I wrote and it's contradictory. What I'm trying to say is that you don't want to rush things, but you also don't want to get too familiar too quickly, which is super easy in a class setting because the typical club bitch shield is super toned down, if existent at all. Keep your conversations short and make yourself scarce, and you can apply the 7 hour rule over the course of several class meetings, which can be a few weeks or even more if need be. Don't be pushy, for example if you can't sit next to her in class, it's whatever. If you talk to her for a few minutes and then she has to scoot off in a different direction than you, so be it, you'll catch her next time.