How do I ask someone out?

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

I go to the gym pretty often. There's a trainer 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.
She's a beginner's trainer so she's usually either working with a client (can't bother her then) or in her office.

But again I have no idea what I'm doing besides overthinking.
How do I ask someone out?
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