How do I summon the Golem boss?

As you have defeated Plantera, you should have now a Temple Key in your inventory (it is a 100% drop from this boss). If you didn't get the key, nor anyone in your group, you must beat Plantera again if you want to proceed.

Now look in the jungle for orange, indestructible bricks. Even your best pickaxe can't mine it (only the pickaxe looted from the Golem can). That is the Jungle Temple. Use the Temple Key to get inside through the unique entrance.

At the very end of the temple lies a shiny Lihzahrd Altar (sometimes, you can even see it shining in the dark from outside the temple). Hold a Lihzahrd Power cell anywhere in your inventory and right click on the Altar. The Golem has now spawned at the cost of one Power Cell.

List of Golem drops, some of them are PC-Console only.

Power cells are found in Lihzahrd Chests (100% chance) and are rare drops from the Temple-specific mobs (Lihzahrds and Flying Snakes).

If you create a new world and attempt to summon Golem using a Temple Key from another world, he won't spawn. You have to defeat Plantera in the current world in order to face Golem, which means that you can't legitimately summon Golem in a pre-hardmode world (though it is possible using world-editing tools).

Anomaly: Sometimes, in multiplayer, there is a bug that doesn't make the Golem spawnable upon Plantera's defeat.

Golem is summoned by activating the Lihzahrd altar found in the final chamber of the jungle temple. Activation requires one Lihzahrd power cell, which must be present in the player's inventory. The activation automatically consumes one power cell and spawns the golem immediately.