The main challenge with PCB milling is the variation in Z height (due to machine misalignment, uneven stock thickness and warpage). There are two basic approaches to this, each one has its own advantages and disadvantages.
The first approach is to use a pressure foot attachment that rides on top of the circuit board and ensures constant milling depth.
The second approach is to probe the PCB and correct the uneven Z height with a software program such as AutoLeveller.
No matter which method you choose, you will need appropriate cutters for trace isolation, copper removal and board routing. Think & Tinker has a good selection of those: www.precisebits.com.