Seriously, how is this done I just dont get it... I can only throw people to the ground as of now...

Speed, momentum and power. You just have to know how to control your Tori. If you play more you'll get much more better. Dismembering Uke is easy. Dismembering yourself is even easier. So maybe just experiment and play around for a little and maybe you might just do it.

I have tried for a couple hours now and the best i did was a VERY slow suplex beheading Uke xD

well just keep trying, stay away from sychronisation too (not sure if i spelled that right) but just try lifting off with one foot, kicking with the other. or turn your body into hits, rotate chest. thats the key of toribash.

I dont know I can barely get off the ground and jump to do a head decap and the like.

Like flubbah said. just keep trying, learn how to control your tori.

You're not gonna learn in one day. It'd take at least a month of hardcore gaming. Just be patient. You gotta try new things. You're improving even though you might think you aren't.

What DeeJayy741 said. Practice makes perfect, and you don't turn into David Beckham playing football for two days.

Also it depends on what videos you're watching, some of them might have the dismember threshold lowered or have an instagib mod.

Or you are just watching ub3r 1337 hax vids. You cant really expect being an awesome replay maker in a day. Also id say try some multiplayer before you start playing singleplayer. Its more fun to learn in MP:

U should see some of my first replays... SHOCKINGLY awful