This video from AskTheBuilder gives a detailed approach to convert a 3-way switch, which allows you to turn on a light from two locations, into a 4-way switch.
In addition to the instructions, the narrator gives safety tips and lots of extra information to help you understand what you are doing.
His descriptions and information are geared to an absolute beginner, so regardless of what electrical experience you have, you will be able to complete the task.
The close-ups of the process of attaching the wires to each other and the switch, which you find starting at 3:35, are especially helpful points to pause the video so you can see that you are doing each step correctly.
And remember, you need to install the 4-way switch between your existing 3-way switches for it to work!
The easiest as well as safest way to do this is to use electrical conduit to place the wires in.
The best kind to use are the fiberglass conduit. They are low cost and will practically last forever.