Finding Joker's hideout isn't very difficult, especially that he won't change his location and you will be updated on the distance to the target. Your objective is reaching the Sionis Steel Mill found in the industrial district in the south-east part of Arkham City. Just like before, you can either go by foot #1 or glide and use the Grapnel Gun #2. In the latter case, you should know that you can't fly through the restricted zone in the middle part of the city.
As the industrial district is located on a separate island, it would be good to go across the bridge #1 which will let you avoid many attempts to stay away from the water. Joker's Funland neon will inform you that you have reached the target #2.
During the exploration of the industrial district, be sure to use the Grapnel Gun often, as the lower levels are patrolled by Joker's goons #1. That doesn't however mean that you can feel completely safe on the upper levels, as some towers are also occupied by single guards #2.
Gradually move south #1, towards the Sionis Steel Mill. Be careful when approaching the destination points, as you can come across enemies with firearms on the lower level #2. Considering that Batman's equipment is probably poorly developed at his points, it would be good to stay away from them.
Your goal is reaching the high tower with multicoloured lights #1. Listen to a short conversation with Alfred #2, from which you will learn that you need to enter the Sionis Steel Mill by jumping into one of the chimneys.