I have built several wheels and fortunately, I always had the old spokes to use as a guide for length. It just requires patience and the ability to follow a set of instructions. Obviously my wheels were not as well built as a professional and they took me several hours, whereas a pro would take a fraction of the time, but they were round and straight and the spoke tensions were pretty even when I finished. It probably helped that I'd had a lot of experience truing wheels over the years.
I'd advise taking apart an old wheel that you have and putting it back together, that way it doesn't cost you anything and it doesn't matter if you mess up. When I say an old wheel, not too old that the spokes and nipples might be seized or damaged and with an existing fairly straight rim. I made the mistake of starting to practise on wheels that were a little too far gone to save (although a pro might have managed). With new rims and spokes it is easier but more expensive if you cock it up.
Lots of Youtube videos are available and personally, I usually find it easier to learn a new skill by following a video rather than reading a manual.
Good luck.