Have 8 amiibo
Use a controller that can “desync” (a GameCube controller preferably)
Go to the 8-Player Smash mode
Just move your joystick until the “Player #” or port space pops up.
Click the name part of your port until it says “None”
Remove your controller from the port or desync it
Scan your amiibo
Plug the controller back in
Move your joystick again and now you will be the player in the next available space after your scanned amiibo.
Repeat Steps 5-9 until all the Player spaces are filled with the 8 amiibo
Go to the Settings (where Time or Stock and Item rules are)
Exit that screen and start a game!