Not hard. My first ever build was a SFF using this case:How hard is it to build a SFF PC if its your 1st time buildling?
(it's an outdated design these days though)
In fact, I would actually recommend people new to PC go with a SFF using microATX instead of one of the other form factors. Very few people actually need regular ATX, and I find it kind of a waste whenever I see builds with one video card and a couple of hard drive inside an ATX case. microATX leaves you with options if you ever decide you really need some extras like SLI or a sound card.