If you mean how to edit files in SimE:
You edit it almost exactly the same way as you'd edit a maxis mesh. To clone a custom file, just click "open" (and open the custom file you want to edit) instead of "start" in the clone menu, and you've got it going.
If you don't know how to edit maxis meshes, then I suggest you go look up some tutorials, since there already are plenty of them.
If you mean how to edit custom meshes to get them ingame:
There already are plenty of tutorials on how to make or edit custom meshes. Depends on what mesh you want to use, and what you want to use it for. If you learn how to edit maxis meshes, then you'll also learn how to edit custom meshes by others.
Modthesims has a lot of beginner tutorials. So does GoS. Here at Insim there are more spesific tutorials.
http://www.modthesims.info/wiki.php?title=MTS2:Sims_2_Createhttp://www.digitalperversion.net/gardenofshadows/index.php?board=34.0