Step 1 -Make a custom game with unlimited time and start the match.
Step 2 - Select spectator and fly to anywhere you want to go.
Step 3 - Change to any team. as soon as the spawn in animation starts switch teams to spectator.
Step 4 - Quickly switch teams and select auto-assign. Make sure to steps 2-4 very fast.
Just select a class again to bring your guns up like in the video .