Ancient cities can be found in the deep dark. The easiest way to find a deep dark is to search for a big mountainrange on the surface and then carefully dig down.
Energy Essence can be obtained by killing the warden OR by sacrificing certain mobs. The Energized Flint and Steel can then be crafted by combinding a normal Flint and Steel and Energy Essence in the crafting table.
To light the portal you will need to carefully remove all the sculk veins from the portal, be carefull as this makes sound. Afterwards you will need to right click the Energized Flint and Steel on the portal to light it, this consumes quite a bit of durability.
When entering this dimension, it is recommended to bring a netherite pickaxe because the planet is mostly made of a very hard block called Umbrusk.