I do it and have just learned by trial and error... sadly, more ERROR in the first two years. Poor Jacob! Looking back through pictures of him actually embarasses me at times!!
My suggestion - find a local barber!!!!
Less mess to clean up and from what I have noticed - my kids sit more still when they are in the barber's chair than with me.
Where I get my hair cut, they only charge $3 - yes THREE DOLLARS for a trim. For a full hair cut, it is $15, but SOOOOOOOOOOOOOOO worth it! I HATE the mess and it just frustrates me.
Of course I will do it if the weather is nice outside and I can just sweep the hair off of the deck.
Sorry - probably not the suggestions you were looking for, but in the whole grand scheme of things, if you can find a good barber/stylist that doesn't charge too much - I'd do it and save yourself some sanity and future embarrassment for your children!!!
OK, OK, if you absolutely want to DIY, first get some GOOD clippers for DS. In the summer, I use a #2 guard on the top and a #1 guard on the sides. Don't cut it all one length unless you totally buzz it all off. It looks better blended (at least Jacob's does).
I might suggest getting his hair cut once while you watch and ask questions. My stylist is very helpful to me and that's how I have learned.
Good luck and let me know if you have any other specific questions. My clippers came with a DVD that was pretty laughable. The haircuts in the demos made it look like the video was made 10 years ago!!