Well, kinda hard to answer "how hard" but I have had electronics Bench tech type jobs years ago so I've soldered a lot of things in my day. I think it would be difficult but do-able for someone with zero experience in electronics or soldering, but I would not recommend.
For a non-experienced person I would recommend the Deceare kit or the Elekit TU-879 that uses 6L6s. I have built both and I think that as long as you practice soldering a little till you get good the instructions that come with those should be good enough (especially Elekit, get it from vkmusic.cn).