load up a new singleplayer world
find a place you want to use for the arena (preferably something flat with little clutter)
center the worldborder to the center of the arena and set the border size
make a hidey hole a few blocks under spawn
place two buttons on the side of the walls
go to a prefilled chest generator and make your kit
make something balanced with the terrain around the players
set command blocks to teleport and to summon the prefilled chest (kit)
you've done it
optional: make something in the center of the map to signify that it's the center of the map
