You could start by learning how to use FILT to make logic components. Once you understand the basic concepts you can create your own memory (explained in the first link) and CPU. Then all you need is some way to output results (a screen or number display) and possibly a keyboard for input. Having prior knowledge about how computers function is a big advantage (you should know binary numbers for instance).

The best tool to create electronics is "Subframe Chipmaker", which comes both as a script and as a mod. The mod has more options but I'm not enirely sure it still recieves updates.