Generally, machining plastic parts is no different than machining a metal part. In fact, since plastic is relatively soft, it machines even more quickly.
But, that softness presents its own challenges. As an endmill cuts, it applies pressure against the material. Unlike steel, aluminum or other metals, when that pressure is applied to plastic, the material compresses. This compression makes it difficult to achieve tight tolerances for plastic parts. It can be done, but it may take extra time, patience and additional expense.
But the opportunities can outweigh the challenges. Designers can avoid this additional cost by putting broader tolerances on a part drawing. They can in fact, take advantage of the relative softness of plastics. Tight tolerances are required when parts need to fit precisely with mating parts. Since plastics are more forgiving than metals, parts can be designed with a tighter than normal fit. Then, when the parts are assembled, the compression comes into play and allows the parts to fit, even though they aren’t excessively precise.
Leaving a little wiggle room in design practices can save time and money.