In order to use a friend's or your favourite content creator's Decal, they must first Upload it in the game and then give you the design's Share ID. This is a 12-character code made up of letters and numbers. Once you have it, go to the Garage, select AC Design, and then scroll down to Image Editor. In this menu, scroll all the way to the right to the Downloaded tab. A popup box will then allow you to enter the Share ID.
Now you have the Decal downloaded, head to the Decals tab in AC Design, and select what part of the mech you want to place the Decal on: the Head, Core, R-Arm, L-Arm, or Legs. Once you've chosen, button down to Custom Decals and then select the Decal you want by scrolling all the way to the end of the Select Image menu to find Downloaded. From there, you can position the Decal on your mech and have it as part of your look in your own game.