Is it possible, that you know of, to accidentally cut the hair shorter than the length of the attachment? Like stick on a 3/4" attachment, push at the wrong angle and get a 1/8" patch?
Possible, but unlikely... Keep the trimmer flat on the BACK of the guide comb. If you mess up too badly, saw it all off (it will grow back :)). The closer the cut, the more likely you are to do a clean job of it (because you can feel variation better where you can't see). For a longer cut, cut it more often, as you are taking much less off each time that way, and any misses are less noticeable.
If you do a really terrible job, just dip your head in butter-flavored Crisco, and use a spatula to form a Dairy-Queen twizzle on the top. No one will notice the hair that way.
Oh, and here is a clue - put a towel in the sink before you start - And NEVER believe 'it will all wash down the drain'.
Otherwise, I'd recommend going to a pro. If you live near a military base, you can probably get a regulation cut in jig time (can we say that?), with no mistakes. Otherwise, SuperCuts, etc., can shear you for one week's allowance.