The main point of Magic Shader is editing your player model inside the game. The pack handles the character changes itself, so you can work on your model without leaving Minecraft.
Set up a good workspace
1. Create a flat creative world, or use a well-lit room with a plain background, so your model is easy to see.
2. Set the time to day, or stand in bright light, so the shading on your model isn't hidden by darkness.
3. Press F5 until you are in the front-facing third-person view. This lets you see the face and front of your model while you work.
Edit and review your model
1. Open the options Magic Shader provides. If it is loaded as a shader pack, look for the pack's settings button on the Shader Packs screen.
2. Change the settings that control your model's shape and appearance, one at a time.
3. Apply your changes and return to the world. Walk, sneak, and jump in third person to see how the model looks in motion, not just standing still.
4. Cycle F5 through all views to check the back and sides.
5. Repeat small adjustments until you are happy with the result.
Tip: Change one thing at a time. If something looks wrong, you will know exactly which change caused it and can undo it right away.