Hey mate.
I would just go to her and say what i want. i would say: hey, i like you and, yeah, you know, i enjoyed having sex with you. I want to do that again.
You already have had sex, all awkwardness should be gone. Straight line, go for what you want. Dont text or anything, just say it in her face. do mind, if you approach her on the job, you might want to say you want to go out again and say the sex thing later on. Talking about your sexual desires in a work environment can cause bad things to happen

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cheers and good luck!