Once the sign-ups for Sky Battle Arena open you will type /warp event and leave EVERYTHING from your inventory and hotbar at home. Once there, speak to the sign-up npc to receive your weapons and armor and set your health to 500.

Be sure that your pvp is turned on. You can see if it is on the left side of your screen. If its not, go to settings -> gameplay -> set battle standard settings to always accept.

This event has 3 main rounds. The winner of each round will sit out until a finals round where the 3 winners will compete against eachother to determine 1st, 2nd and 3rd place. Last exile standing wins.