What's the proper way to fill this hole?
Acquire a rotatory impact hammer drill so that you can pocket rebar into the existing concrete. Buy 'high-strength' expensive concrete. If you called a truck for onsite mixing (an unnecessary expense), tell the driver, 7 bag mix. 

An alternative to rebar, for such a small patch, is to use long masonry screws, left standing proud to provide an anchor. E.g., use 3" screws, embedded a mere inch, using at least two per side (quarter inch diameter fasteners, minimum).

Knock down any sharp edges or small 'lips' in the hole, as it will crack along them later if you don't. I'd square off that part where it slopes down into the hole; it's not going to be thick enough there if you don't. Remove all unsound or lose concrete.

Before you pour, settle the substrate by soaking it with a garden hose. While you spray it, hit the sides of the old concrete to clean them of dust and debris. I like to slightly undermine what's there, so that it all locks in like a tetris piece. Wait until all standing water has dispersed.



There's what looks like a drain inside.

Insure the drain is 'properly' capped (plug it with empty concrete bags, leaving a little room for concrete and encapsulate the whole thing with your pour), this could be the root of the problem, or it's at least the reason a patch was slapped at it, in the first place. Do a better job anchoring it this time and you should be fine, even if the drain still weeps a bit.