There are a number of plans for DIY CNC machines out there, some free and some available for a small price. I chose to make the JGRO 3-axis router table, which can cut wood or plastic. This machine should be large enough for most guitar wood-working and the plans freely available from cnczone . A major reason for choosing this design is the support available from the forum at www.cnczone.com. The forum is friendly, knowledgeable and helpful, with a lot of people who have already built these machines and overcome the sorts of problems you might encounter.
This machine is made from stuff you can mostly get from your local hardware shop, but with some specialised items such as big stepper motors and their control electronics. You will need access to a table saw, band saw, router and drill-press, as well as various drill and router bits. I was lucky enough to be able to use a relative's band saw, but I bought a cheap drill-press from www.screwfix.co.uk.