The first step was figuring out the scale. I used a mostly round metal bowl for the rounded top, which had a diameter of 6" at the mouth, so 6" was the logical size for the rest of the cake. I used packet mix cakes, 4 of them, to make my life easier, when I was both working full time and making an Ewok costume in the same week. I baked the cakes over two nights and cut the tops of the three rounds, and trimmed the "head" to be a bit rounder.
The four pieces were crumb coated using white chocolate ganache, then stacked together and secured with three skewers before going in the fridge overnight. I stacked them on a slight angle, the way R2D2 is when rolling along. I used the natural skew-wiffness of my leveling to create this angle. The next night I smoothed out the ganache and wrapped on white fondant for the "body".
I coloured some more fondant grey and cut out a circle using a pot lid that I smoothed on the top. Then the fun began. I coloured some blue, cut out a circle, then another circle from inside that. The smaller circle went on top of his head, the outer circle I cut into about 8 pieces and spread 6 of them around the centre circle. Using leftover grey fondant I made the sticky out bits that go on his head, and used a short piece of skewer to secure it in the appropriate places. The eye was a rolled ball of chocolate fondant cut in half with another piece of skewer to secure it. I cut out a bunch of blue and grey pieces and matched them to the model R2D2 from the toy box. The red light is just a teeny piece of red fondant.
For legs I made a slice in double quantities and left it in the fridge overnight to go solid. Using masking tape I made a pattern of the height and angle of the legs and feet. I put this pattern on the slice and cut them out.
I crumb coated these, with butter icing this time, and left them in the fridge overnight, before covering them with white fondant in the morning.
The last step was assembly which I did at the venue. I took along some white buttercream to use as glue (pure buttercream goes whiter with just a smidge of black gel colouring). The body of R2D2 was all sitting on the top of a cake base which a balanced on top of a small squat jar, using icing to prevent it sliding around. Then I propped the legs in place, leaning backwards, and used a blob of buttercream on each side to keep them there.