“Take it slowly” is correct. “Slowly” modifies “take,” so it’s an
adverb, so you want the “-ly” on the end.
That said, “take is slow” is idiomatic, which means it’s what people
actually say. I’m not sure I’ve ever heard anyone say “take it slowly.”
So there are your two options, with a justification for each. You get to
choose.
For what it’s worth, my grandmother told me I could never be embarrassed by
saying something right.
You’re probably wondering why she didn’t say “rightly.”
https://qr.ae/pypixb