Here is a List of usefull Tools along with guides on how to use them that you will need to create and install custom skins:
LCS Manager – Custom Skin Installer
Obsidian – League WAD File Extractor
Scavenger – BIN Editor
Paint.NET – DDS/Texture File Editor
Model Resource – Free 3d Models
Ultimate Unwrap Pro – Model Viewer
Creation Guides on how to use the tools will be added over time!