The Y-axis is similar to the X-Axis but sandwiches the Y axis frame piece. It also holds the Laser and Motor, which moves the Y-Axis (and laser) from side to side.
Start by finding the plastic sides and hardware. Here are some pics to help you out.
You should have 6 small screws left to attach the laser and the motor. This part is pretty simple... just line up the holes and bolt them on. One thing to note is that the laser bottom edge lines up with the edge of the plastic. The vertical slot holds the screw which goes into the top hole of the laser (closest to the wire plug)
Check out the pics to see where the wheel assemblies go. Just sandwich the wheel between two spacers and the plastic sides. Tighten the nut for each assembly and slide it all onto the Y-Axis frame piece.
You should be able to slide the asembly from side to side... fun, right!?
Next step we will get the belts all set up and do the final mechanical touches.