THAT is a really good primer on resurfacing that Neal provided. I did this kind of job on some elevate back stairs and had to build a fully framed set of forms. The Quickrete products are excellent ... but beware they do set up fast in the sunlight so moisture content is crucial.
They glossed over the use of bonding agent from the bottle. I always use it when trying to fix old cement with new. Just remember to let it set up before applying the new Quickrete.
Here is a large surface version to review:
Also, you might consider sealing the finished surface to help repel water. This will retard cracking in the future.