It;s actually really easy to do, but it does take practice. If you have a long haired dog, or an old doll, that'd be good to practice on. Even when I first started doing it I still had to deal with a couple bad haircuts (my hairdresser would scold and scold me) but now I can do it really fast, and well, enough so that I've gotten very surprised compliments from my hairdresser.
Unless your hair is really short, long hair is incredibly easy to cut.
I think it's easier to do wet, and then touch up after I've dried it.
I always find bangs to be a bit of a challenge, but they grow out quickly, so if you mess it upthere's not much harm done. Bangs can be straight across, thick, small wispy, or diagonal and fading into your hair.
Decide how you want them to look, brush out a small bit at a time, and cut.
Boys are really easy to cut. Just imagine a horseshoe shape going along the top of their head, with the straight part at their face. Brush this imaginary horseshoe to the front and clip all around it with shears. Then cut this horseshoe a little bit longer.
Boys without shears:
My son does not like the clippers, so I have to scissor cut his hair. This is really hard to do, so I would only recommend it if you have been practicing cutting a lot.