A CNC router machine is probably the most useful tool a hobbyist can own, but the price for a CNC machine on the market is way more than the average hobbyist is willing to spend. You can build your own CNC with very basic tools, little knowledge of machinery, mechanics, or electronics, but be warned, these machines are inherently dangerous, so wear the proper protection and use common sense. At the very least, read the instructions and precautions on every tool you use.
Now, a CNC stands for Computer Numerical Control, but that is gibberish. It does hint you to the fact that something is controlled, and probably by a computer. It's really a mechanism that interfaces to a computer to control its movements, similar to a robot, but this machine has a specific function. It cuts or shapes things with this control. In essence, a computer precisely controls a cutting tool, like a router for instance, to cut materials such as wood, plastic or metals into shapes only bounded by your imagination.
This video tutorial will show you how to make the y-axis rails and transmission nut for a CNC router. Now you can cut the correct length for the rails.
To cut the correct length, simply place an aluminum angle next to the gantry sides with one on the inside dimension of one side. Mark the other side a smidgen smaller than the length. You don't want the angle to bow and the y/z axis assembly will squeeze it together enough to press the angle into those beautiful chamfers. Do two of these, one for the top and one for the bottom of the y-axis rail support piece.
Ok, now onto the transmission nut. You will have the challenge of doing some precise drilling here. Like with the z-axis lead screw, the y-axis screw is pretty much the same. All of the holes must align somewhat perfectly.