A few months ago, I purchased a RepRap 3D printer. If you've looked at the RepRap printers, you'll be aware there are many variations. I chose a kit from dx.com that is manufactured by Heacent. I'll go through the pieces of putting it together and my recommendations on the best sequence of putting it together since the directions are sketchy at best.
I wouldn't necessarily recommend getting the kit from Heacent, or buying from dx in this case. If I'd bought the kit directly from Heacent, I could have saved a lot of money (maybe $100). Also a few of the printed pieces were pretty sketchy as well. They weren't well printed and didn't look very solid, though they've all worked fine so far.
If you've played with LEGOs or done any sort of kit assembly like IKEA, you'll probably be fine with the assembly of the frame. As tempting as it may be, though, the frame is likely the last thing you should work on. My experience leads me to believe you'll get more enjoyment and better productivity of your spare time by testing some components and the electronics first.
At the end of the assembly of my printer, having plugged everything in, and screwed all the wires into the terminals on the Melzi Ardentissimo printer board, I found I couldn't upload the Arduino firmware that would read the Gcode files and print the object. So, my recommendation for the first thing to do is to find out if your board runs and is able to get a copy of the firmware. Ultimately, the Melzi Ardentissimo is just an Arduino board specialized for 3D Printing.
The kinds of errors I was getting trying to upload the firmware indicated that my board was missing it's bootloader. Now, it's possible to run the firmware without the bootloader, but it's much easier to upgrade the firmware when fixes come out or if you want to customize your printer in some fashion (new material nozzle etc etc). Since this is an Arduino at heart, you can program the bootloader as you would any other Arduino, well, almost. The ATMega chip used on this board is a 1284p (some Melzi boards use the 684p) and can't be programmed using the USBIspTiny. Any other Arduino can be programmed as an ISP however, and I just happen to have a few lying around.
I'll write up how I'm doing this in another post, but there are more than a few tutorials out there on how to do that.
Once you get your board electronics working, I would next recommend labeling the wires from the hot bed and from the hot end. The kit I got, the wires from the hot bed and the hot end were two red wires and two white wires each. The two red wires are for the heater and the two white wires are for the thermocouple. Not explained anywhere in the documentation and as I found out, quite annoying to track down once you've wrapped the wires up for bundling on the frame. Additionally, for the hot bed, there will likely be a polarity you will need to keep track of. Tough when the wires look the same.
Finally, the part I enjoyed quite a bit, assembling the frame. Make sure the surface you're working from is level. It makes other bits of the assembly easier if you do this. Some kits come with frame measurement pieces to make sure the distance between the vertices are precise, this one doesn't. In retrospect, I would recommend making your own as it makes assembly easier. Additionally the kit I have doesn't provide anything like jam nuts or any other sort of way to prevent nuts on the inside of the threaded rods from getting loose. Jam nuts or serrated washers are all things that can be picked up fairly easily from your local hardware store, though you may be required to put up with zinc'd steel instead of stainless steel. Additionally I would recommend getting some extra threaded rod as the bottom rod provided is a little short.
That's all for now. I'll write more later.