Hi i would like some help to convert a sims 2 cc to sims 3, its for private use ONLY and will never be posted. I have tried converting it myself with a youtube tut but i never managed to finish it. Its for my girlfriend who has nostaliga about a specific mermaid tail, but the only cc out for download is for ts2. So i would really like some help. And maybe that someone explain what exactly im doing, im just following the video and im so lost.

You're kind of in the wrong section - You'll want the TS3 modding area for this. You'll likely find some tutorials there.

Tutorials: https://modthesims.info/f/619/
Creation area: https://modthesims.info/f/593/

Not sure there are any tutorials on 2t3 body mesh converting spesifically, but you'll probably find some that show adding joint assignments using OBJ meshes (mesh format without joints).

You'll likely want a tutorial that shows how to extract TS2 meshes, plus SimPE and either Milkshape or Blender, and of course whatever is needed for TS3 (TSR Workshop and/or S3PE, possibly some plugins and whatnots).

(I don't have any experience with TS3 creation, other than having extracted stuff from TS3 for use in TS2, but if you need help with extracting from TS2 I can help with that part).