All the info you need on creating custom content, including a lot of tutorials, can be found at Mod The Sims (
link). However, you need to be really good at 3D modelling and have the right programs for it if you want to create stuff from scratch or modify the shape of existing objects. If you just want to do recolors, then you need a good graphics editor like Photoshop. You also need a program called SimPE to edit the package files. If you go over to Mod The Sims and have a look there, it tells you all you need to know. But it's not easy - I had a go at just modifying the size of some hoop earrings to make them bigger, and it took days to get them working right!!
