What to do with cash flow?

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Forgive me if this has been covered elsewhere. I searched but couldn't find the answer to my specific question.

I'm looking to buy my first rental property with a down payment/closing costs from my primary residence's HELOC (currently 4%) and a mortgage (3.125%) for the rest. The properties I've got my eye one can potentially cash flow around $175-200/month.

Longer term, my goal is to use gains to continue buying rentals.

My question is this -- what should I do with the cash flow to get ready to do the next property down the road? Put it into the HELOC? Make extra principal payments on the mortgage? Other options I'm not seeing?

Thanks so much for any advice!!