Assembly is straightforward but takes a while. Comes with all the tools you need. Everything is metric. I recommend labeling the screw packets before starting so you know which size is which.
I ended up tightening the drive belts after initial assembly, which is tough to do. I used vise grips to help. Do not overtighten the belts however, because it can cause strain on the servo motors.
I also added a couple washers (of my own) to the pen mechanism to reduce slop. On either side of the arm that raises / lowers the pen. If you do not add your own washers, you can also use the white plastic spacers that come with the plotter.
Ensure that the pivot screw that holds the drawing arm to the upper assembly is tight however. Horizontal slop will lead to impersice drawings.
There are two software packages, one is Windows only (Benbox) and the other is cross-platform (mDraw). I've only used mDraw. Each ships with its own firmware I think.
Their software is crappy but good enough as a starting point. It's easy to roll your own software in any language that can connect to the serial port. Baud rate is 115200.
Their firmware is also on GitHub (and a modified version in my xy repo). It's super basic so that's something else you can tweak if needed.
I usually use this paper, I prefer the vellum surface:
http://www.strathmoreartist.com/draw-bristol/300-series-bristol.html
And these pens:
From the local craft store. Anything similar should be fine. The pens come in different sizes. Keep the cap on the pen when not in use, they dry up fast.
I use double sided tape underneath the paper I'm drawing on. Initially I just taped down the corners but if the paper lifts up off the surface of the table, even just a little, the pen will leave stray marks where it shouldn't be marking as it raises / lowers.
The pen servo takes an angular position, 0 - 180 degrees. You basically set an up value and a down value in the software.
The important GCodes for the default firmware:
G28
Go Home - move both axes until limit switches are hit and set internal position to (0, 0). The firmware doesn't differentiate between the min and max limit switches, which is dumb.
G0|G1 [X<xpos>] [Y<ypos>]
G0 and G1 are identical. Move in a straight line to the specified (absolute) point.
G0 X10 Y10
Go to (10, 10). Units are millimeters. Working area is about 315 x 380mm.
G0 X100
Go to (100, Y) where Y is the current Y value.
M1 <angle>
Set servo position.
M1 0
M1 50
The firmware can only handle one GCode at a time. So you have to wait for an OK response before sending the next command.
Windows installation is pretty much by the book. I found that loading up mDraw was a great way to load the default firmware onto the Arduino. I recommend using mDraw to print the concentric circle test file. This should be a good test to ensure there is no slop in your setup. The circles should be crisp and precise. Any wobbling in the line or deformation in the circle means that the plotter needs fine-tuning. A properly printed test file should be obvious, but take special note to ensure that lines meet at the end of each circle. There should be no gap in these circles.
The X,Y Plotter will NOT work on El Capitan regardless of what the MakeBlock site suggests (as of 2/24/16). El Capitan requires all drivers be signed and at the time of this writing there was no officially supported USB driver for El Capitan and the Orion Arduino board.
If you are using Yosemite you can install the driver following these directions: https://github.com/Makeblock-official/Makeblock-USB-Driver