If possible, put a piece of plywood on one side as a backer. Run some tapcon screws or similar half way in, all around the opening. This gives something for the mix to hold on to. Make the mix very dry so it won't run all over the place.
Assuming this is just a cosmetic repair (you don't plan to put heavy load above the patch), it is a simple matter of cleaning the hole by brushing to remove loose debris, mix up the concrete, place. I agree with the post about mixing it up dry, much stronger, less likely to crack. I would not be too concerned about bonding to the existing concrete, assuming this is a cosmetic fix rather than structural.
I would avoid hydraulic cement, costs more, sets very quickly, no aggregate, so is not as strong as normal concrete.
A forum community dedicated to Do it yourself-ers and home improvement enthusiasts. Come join the discussion about tools, projects, builds, styles, scales, reviews, accessories, classifieds, and more! Helping You to Do It Yourself!