Instructions
Go to a "Minecraft" skin website and search for a Deadmau5 skin that you like. Download the file and save it somewhere convenient.
Load the official "Minecraft" website and sign in to your account.
Click the "Preferences" tab in the top left corner.
Click the "Choose file" button underneath your character display. Select the download file from Step 1.
Select "Upload" and the page will refresh, your new Deadmau5 skin showing in the character display. Load "Minecraft" and enjoy looking like Deadmau5.