Pre-Step 1) Ensure permissions are all set or people are OP.

Step 1) Type /spleef arena create <water/lava> <diameter> <arena_name>
- <water/lava>: This is what will rise in the arena
- <diameter>: This is the block-by-block size of the arena. E.g. 30 -> 30x50x30 (30 blocks wide, 50 blocks tall, 30 blocks in length). All arenas are 50 blocks tall
- <arena_name>: This is simply the name of the arena
Step 2) Create your arena using concrete blocks; feel free to use worldedit. Just don't break the barrier or bedrock blocks surrounding the arena with worldedit.
Step 3) Type /gamemode survival
Step 4) Wait for 2+ players to be online
Step 5) Enjoy!