Research...
Reference pictures were the first thing I went for, something I asked myself was how well do I actually know R2? Short answer is not as well as I thought I did! The first thing I learned is no matter how many times I have watch the Star Wars Movies or collected action figures there were loads of small details that I simply didn't notice until I looked at him in detail and no doubt at the time of writing this entry there are still bits that I haven't noticed that will become clear when I break down the parts.
..Research..
As you can see from the images above, if you want a replica of R2D2 then you need to copy it down to the smallest detail which can be really tricky if you aren't 100% sure. On the stomach.net forum, there is a page called Droid Wiki which gives a detail run down of all the parts of the droid many of which can be applied to builds that aren't of R2D2 which for anyone who is reading this who isn't building R2 but a different droid is great news. Here are some drawings I have done (I told you I'm no artist!) of different parts of my droid. These, at the moment, are simply ideas I had that I thought may make my droid easier to work on such as a removable mid-section.
..More Research!
I came up with this idea when I was deciding if I wanted a hatch in the back of my droid. The hatch seems to be a common thing among droids as it offers an easy way to get to the electronics in the body. Upon thinking of this I wondered if I could actually have a removable mid-section where I would remove the dome and have easy access to the cables going to the feet motors etc that I can disconnect and slide out the mid-section. I would only need to do this for either large-scale repairs, adding or removing parts of my droid, so I don't think this would be a smart move however I do plan to add things to my droid at a later date.