How do I ask somebody out?

How do I cold approach a girl without coming across as creepy, catcall-ish, douche, etc?

I go to the gym pretty often for the past year. There's a trainer who's been working there who I find very attractive. How do I approach her and strike up a conversation or ask if she wants to grab a coffee or something?

I hear asking a girl anything without her even knowing me is like 99% rejection.. as she doesn't even know me. The problem is, she's a beginner's trainer so she's usually either working with a client (can't bother her then) or in her office. (Yes I've never interacted with her once out of an entire year). I'd like to get to know her and start conversation, but I can't just poke my head in her office (where she usually is) like a weirdo and say "hey".

So either I can just straight up ask her, "Wanna grab a coffee sometime?" Or I can walk by her office and start a conversation about fitness certificates or something. I have no idea.

Any ideas are greatly appreciated.
How do I ask somebody out?
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