I've only done a little patching on a wall here.... I asked around some before I did.
The hardest part is getting it all clean. Must be free of loose crap, grease, oil, all that. Just as important.... you have to use cleaners that won't leave allot of residue. Simple Green or CitriSolv rinse away pretty easy.
Next part.... leave the area you are patching damp. Let some water soak into the existing concreate. Concrete doesn't "dry". It chemically reacts and it needs water to do this. If the existing concrete is dry it will suck the water from the patch and stop the reaction too soon. That's probably why it's powder below the skin.
Keep the patch damp after it's applied. Dampness will make it set stronger. Cover it with plastic or something to keep it damp. Yes, once it starts setting you can mist it with more water if need be.
If it's a larger crack in a wet area, consider "hydraulic" concrete. This stuff will work even under water. (That's what I used to patch my wall... tought stuff.) Expensive but was worth it for me.
For fine cracks a liquid crack sealer might be better. That will flex and not just pop back out.