Try the tips, tutorials, & tools section under card modelling.
Basically, I start with as many drawings and photos as I can find. The best get pasted into a drawing/graphics program (basic PowerPoint works) and then enlarged or reduced to a consistent scale. That allows me to measure dimensions.
At this point, it's time to get out the pencil and paper. I sketch out the model and break it up into boxes, cylinders, conic sections, etc. For complex shapes you really need to use a 3-D graphics program (or just be prepared to do some cut, fit, and redrawing, repeating as necessary). Once I have an idea of what the parts are, I go back to the scaled photos and calculate the dimensions of the resulting flat parts (some basic solid geometry) and draw them up.
Most 3-D graphics programs will do this for you - called unfolding the surfaces of the model. For details here, you'll need to refer to the documentation for your 3-D program.
It's both easier than it sounds and takes longer.
Yogi