I use a CAD program called Rhino 3D. I start by creating a 3D model of the part. I use the 3D model so that I can combine all of my parts into the finished product and check the finished product to make sure everything will work together. In reality, you can often use a 2D CAD program to design your parts. For those of you with Rhino, here's the Upper-Mount.3dm file. Bigger...

The next step is to decide what machining operations you're going to perform, and create contours or other supporting objects that will be used by the CAM program. I normally use features from the 3D model to create the CAM objects. In this photo, you can see a green rectangle which represents the material which will be machined. The red outlines are the outline of the part. The blue circles represent holes which need to be drilled. I also like to double check the cut, so I offset the red contour diameter of the endmill I'll be using. This lets me make sure that the cutter won't interfere with anything. I then select all of the objects (except the purple lines) and export them into the Upper-Mount.dxf file. Bigger...

I use a program called SheetCam to generate my G-Code. It takes a DXF file as input and generates the G-Code. Incidently, I'm running SheetCam on my linux Ubuntu distribution, by using wine. SheetCam uses a notion of "Processes". In this particlar job, you can see three. The first process center drills each of the holes, the second process drills the holes, and the third process machines the outlines. SheetCam also has a rather nifty feature called "tabs". These can be seen by the short blue segments in the red outline. The tabs are small amounts of material which get left behind to leave the piece attached to the material. You'll be able to see the tabs in the G-Code simulation in the next photo. Here's the SheetCam job file: Upper-Mount.job along with the generated G-Code file: Upper-Mount.nc. Bigger...

I then take the G-Code produced by SheetCam and use CutViewer Mill to verify the G-Code and ensure it's going to produce the correct part. You can see the tabs in this screenshot. CutViewer will also check for collisions (i.e. trying to rapid through uncut material). It doesn't have any capability to check for collision with clamps (and I've run into my clamps a few times :) You can also do cross sections and take measurements from the simulation as well. CutViewer Mill is also being run on my linux box using wine. Bigger...

Here you can see the finished part (in the top right corner of the picture) along with some other pieces. The shocks and ball ends were purchased. Bigger...