How does one approach a fit chick?

You say "Hello"

but they have bubble butts

What are you gay?

"how does one..." is often a terrible way to start a question lol

The question also lacks details... where are you? Who is she with? etc.

If it is a woman walking on the sidewalk and you find her attractive, then you just have to go up to her and make small talk. Don't back down easily and be friendly, charming and somewhat polite (polite that you come off as nice, but not a pushover).
The problem with these cold approaches is that you guys have little in common at the moment (whereas at a gym it is easy to spark more specifically related conversation). You also have to have balls to go up to people and initiate conversations and handle a potential rejection.

She prolly insecure as fuck like everyone else who works out

So just exude confidence and make that kitty purr

Just do you....

dont try to be somebody youre not

just be yourself and if she finds that attractive, you put it in her butt

this

im at a gym, im an aspiring young lifter on a gain train

I've also had success with hi

Greetings has never worked though

or welcome to my humble abode.