Arena for the Wall of Flesh
It is highly recommended to create an Arena for the Wall of Flesh since the general terrain down in the Underworld is very hazardous and the area has various elevation obstacles like the ruined buildings and fluctuating terrain heights.
The main feature of the Arena you should build is just a simple flat terrain. Most of the time, just Wooden Platforms are used so a massive amount of wood is required.
Quick Tip: Carry wood instead of platforms. Platforms can only stack up to 99. While the wood resource is more flexible (you can build stuff with it) and can stack up to 999.
If you feel you can put in extra effort, do two layers. One low and one considerably higher where you will be relying on the grappling hook to get you up there.
The fight will consist of it moving left or right depending where you summoned it. It will spawn at the side in which you summon it. For example, if you want it to move from left to right, summon it on the left side of the map. To make it go from right to left, summon on the right side.
Since it will continuously move, you will die if you reach the end of the map and you are cornered. Using a Magic Mirror to return home will also kill you. If you wish to avoid dying and escape, Exit the game from the menu.
Quick Tip: To give the maximum amount of battle time, summon it NEAR the edge of the map NOT AT the edge of the map. Summoning it at the edge may cause you to waste the item and the Wall of Flesh won't be summoned.
IMPORTANT! Killing the Wall of Flesh plummets the world into Hardmode. A lot of things will get stronger and much more difficult but new items and challenges will spawn. DO NOT kill the Wall of Flesh unless you are confident that you can survive in Hardmode! DO as much as you can to make your character the strongest it can become in Easymode. The preparations for fighting the Wall of Flesh should also include going into Hardmode.