Valve amps are crude, simple, and dead easy to build - the only problem is the cost - transformers are
EXPENSIVE items.
There are countless sites out there with designs, or you could simply copy an existing (old) commercial design - schematics for almost all of them are freely available.
My main concern though is why do you want one?, the majority of guitar players use solid state ones, and even ones who use valve ones on stage often use solid state ones in the recording studio (due to their much higher quality). If you're wanting to play over-loud heavy metal though, you NEED a valve amp, and probably a Marshall one