I've got a Solidoodle 2 3D printer which has a build envelope of 150 x 150 x 150mm so the first thing I did is cut up the bonnet into to 150mm cubes (I should point out I didn't have the printer at this point so I couldn't test any of this), gave it some extruded sides for rigidity, first mistake. The printer can't print on thin air so I would have to build up the top extruded side at 45° or beter yet cut the squares at 45°. so I cut all the squares diagonally and then gave them 1.75mm depth, second mistake. 3DS Max 2011 (which is the design tool I'm using) gives depth along each segment's normal (imaginary line perpendicular to the average face direction) instead of uniformly in one direction (x, y or z axis). So start again, give the bonnet depth first, then cut, then make the extruded sides (and give them depth as well).
Once all this was done (just the bonnet so far) I had to rotate each segment for printing, export and convert into gcode, given the trouble I had creating the segment I only did one and waited for my printer to arrive. When it did it was pretty straight forward, plug the USB key in, use the software provided to turn the exported .stl file into gcode and print!
The first print worked fine except the printer didn't handle the 45° angle very well, it printed it but it had trouble and it wasn't true (was actually a bit saggy). I need all sides to be true or when I attach them all together it will be misshapen. So start again!! This time I gave it 5mm depth, cut it into 150mm squares and only extruded the sides by another 5mm (as being 5mm thick it's sturdy enough, the sides are only to enhance the accuracy when I stick them together, also it's much much much easier and faster doing it this way as well).
Another problem, converting it to gcode creates rails the print sits on (used to keep it stuck to the print bed as as it cools it wants to distort) and as I've used the full build envelope it looses the sides, also the bed is heated from the middle so the outsides are cooler and the edges curl up too much, but the last straw was the printer got to 109mm, threw errors and wouldn't go any higher. I'm sure there was just some adjustment required to fix this but with the rail and the warping I decided to start yet again, this time I did the boot as I had had enough of the bonnet by now.
Something I found out along the way was you can spray hairspray onto the bed to help keep the print stuck to the bed, also (which I have yet to try) use a pane of glass (sprayed with hairspray) on top of the bed (kept in place by paper binder clips).
The final solution that worked:
1) Give the object (boot) 5mm depth
2) Cut it into 105mm squares
3) Extrude the sides by 5mm
4) Export to .STL, convert to gcode (using the rail settings)
5) Heat the bed up to 95°
6) Spray the bed with hairspray
This works pretty well, occasionally the edges still peel of the bed but most of the bottom is still straight so there's enough to stick them together accurately.