Let me try: Boris the Badd. Nah, I don't want a fighter. I want a barbarian with 3 levels of rogue. He started in the wilds, learned to use some stuff then came to the city. He liked strong drinks and soon ran out of money. Fortunately, a quick, skinny guy had a proposition for him. There was a guy that was bad for his debts. This guy needed to have a lesson taught to him. Boris helped his new friend teach the debtor a lesson and he got a little more coin. He kinda liked the work too.
Now, he has been working with the thieves guild for a couple of years. He does a little strongarm work, as well as hiding in shadows and waiting for drunk nobles to come walking by. They are often quite generous with their "tips" to be sure they arrive home safely.
So, what do I know about Boris? He is strong, but I don't want him too strong. He is also a bit stealthy. The better to mug people with.
So, let's see he starts with a 16 str. We will bump him to a 15 Dex and a 14 Con. Everything else is 10. He will start with 16 skill points as a Barb, he will pick up 24 more with rogue levels. Dang! That's a lot of skills. Oh yeah, he is human, give him 7 more skill points. We don't need that many, but we will max out intimidate at 7 ranks. We will also give hide 7 ranks. That's 14 ranks out of 47. If we ever flesh him out, we will have a lot to play with.
Str: 16 = +3
Bar1 = BAB +1
Rog3 = BAB +2
Total bonus = +6
Let's give him Power attack, improved grapple and improved unarmed strike. for his 3 feats.
Leather armor and a club.
Dex 15 = +2 AC
Leather = +2 AC
AC = 14
HP: 1d12 (maxed) +3d6 (ave) +2x4 (Con) = 12 +10 + 8 = 20
Now, I have a decent base NPC that can rage to be a little more effective. There is room to flesh him out and it took me much, much, longer to type than to figure it out.
What did I forget? Doh! Saves. I would have to calculate those on the fly. But, right now I will say Fort: +2 (bar) +1 (rog) +2 (con) = +5, Ref: +0 (bar) +3 (rog) +2 (dex) = +5 Will: +0 (bar) +1 (rog) +0 (wis) = +1.
So, for low level it is really easy. For higher level NPC's, equipment is harder for me to track. Also, this NPC is odd in that he isn't something I have ever considered before. Could I do it without missing a beat? Probably not. It would take me at least a minute or three. A more complicated NPC would take a little longer and I would play a little looser with the numbers. But, my players hardly ever notice when it comes up in the game.