There are numerous ways to make printed circuit boards by applying a printed mask and etching them in a chemical like ferric chloride, but using the router can save a lot of time once you get over the learning curve and find the correct parameters. Additionally, you can mill the board to shape and drill holes for thru-hole components in the same process.

Milled circuit board

To get started, you will need to generate your circuit board layout in Eagle and then download the plugin (click latest release).

In the board view, open the plugin and you will find a control panel similar to the following:

Preview in the PCB gcode plugin

Finding the optimal settings take some experimentation, the screen shots above should provide you a starting point to work from. Use the generic gcode style and spindle speed of around 15,000RPM. You will have to manually set the speed in your gcode. Be sure to drill your holes before milling the traces or the drill bit can rip pads off the board.