To start the game the host should /tp up through the barriers above the spawn point. There is a sign that says "Start Game". Once all of the players are there you should click the button directly under it and the game will begin.

Fight with swords or break one of the tnt blocks to gain a snowball which can be thrown as an explosive. Those who are not holding tnt will have a green trail.
Those who are holding tnt will not have the green trail but the tnt will appear the be smoking as if it were about to explode.
There are three places to collect tnt to throw at your fellow players and there is also a small parkour with landing pads lower into the map.