If you're asking that question, I'll wager that it will be quite hard.
I've had to rebuild some tube amps that we use for our RF power supply in the lab. I could certainly wire up an audio tube amp from a circuit diagram. To design a good sounding tube amplifier would be a totally different story. I'm sure I'd have to do plenty of reading and prototyping before I'd get anything that sounded reasonably good.
Also, a one-off tube amp is going to be quite expensive. For electronic components things get radically cheaper as you buy in bulk. The unit cost of some items will be double the unit cost if you're buying one as opposed to if you are buying 10 or 25 or more. It may not be prohibitively expensive, depending on what exactly you're building.
Also, with a tube amp you can be dealing with potentially lethal voltages. If you haven't built any electronics or done a fair amount of wiring then a tube amp is, IMO, a dangerous first project.