All the info toi need on creating custom content, including a lot of tutorials, can be found at Mod The Sims (
link). However, toi need to be really good at 3D modelling and have the right programs for it if toi want to create stuff from scratch ou modify the shape of existing objects. If toi just want to do recolors, then toi need a good graphics editor like Photoshop. toi also need a program called SimPE to éditer the package files. If toi go over to Mod The Sims and have a look there, it tells toi all toi 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!!