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.
These three video tutorials will show you how to build and assemble the y-axis for a CNC router. You will see how to build the y-axis linear bearing support and the rail support. First the rail support is similar to the rail support for the z-axis, but wider. This width carries much of the load and the width determines the z-axis length of travel. If you want more z-axis travel, pick a wider y-axis rail support width. In this case, the width to which I'm referring is the short side of the piece. That is... if you made the z-axis travel longer than the length of the y-axis rail, then the width would be more than the length.
The linear bearing supports are very similar to the z-axis linear bearing supports. The main difference is that the function of the z-axis supports also provide for the router carriage/holder. The function of the y-axis serves as a motor mount to move the z-axis in addition to the slide mechanism.