How do i backstab someone??

Sneak up behind them (invisible/hide in shadows etc) then attack them. You'll have to be using a dagger or a short or long sword. Other weapons (to my knowledge) cannot be used for a backstab. You'll see a little message telling you wether it worked or not and a nice little pile of extra damage... up to 5 times as much.

Oddly enough I use Yoshimo (lev' 19) with a long sword. He's never once connected. I'm beginning to wonder if you've got to use a dagger or short sword. Maybe I'm just unlucky....if I was in a marching band I'd have a piano...I just know it.

Make sure you're not visible and behind your target. Also some thieves cannot backstab due to the kit they are equppied with. Can't remember if he is, but if Yoshimo is a swashbuckler he will not be able to backstab. Other than that you should be able to do it. My thief (level 15 I think) regularily deals out 50 or so damage with a backstab using a +3 longsword. I know for sure that 2 handed weapons don't work but the shorter stuff should (including katanas and scimitars).
Anyway good luck

Daggers, short swords, long swords, katanas, scimitars and hide in shadows BEHIND your target.

Charlie, once i made an awesome backstabb with yoshimo and the celestial fury (about 120) so sure he´s able to do it (grim avenger, yoshimo is a bounty hunter).

Just try it hard !!

Yes, Yoshimo can be a pretty good backstabber, but he is nothing against my kensai/thief that has done 264 damage with one blow (no strength modifiers), and rarely does anything below 65 damage.