A basic tutorial that will probably be enough for most modders. Read this if you want to use Blender to modify existing assets inside a model (e.g. change or delete part of a mesh, change a texture, etc.)
Not for the faint of heart, but I cover a lot of ground so it is worthwhile in my opinion. Learn to transfer assets, including weight transfer and mesh segmentation. I also cover how to add new textures to the g1t, and how to add new meshes to the g1m.