The 
Ancient Compass is the mod's signature navigation tool and the answer to "how do I find the temple" without wandering for hours.
Crafting the Compass
Open your recipe viewer and search for Ancient Compass. It uses Deep Dark materials, so you will need to have visited the Deep Dark at least once, or traded for the ingredients, before you can make one. Craft it as soon as you have the materials.
Reading the Needle
Hold the Ancient Compass and watch its needle. Unlike a vanilla compass, it does not point to your spawn or to a lodestone. It points toward the nearest Ancient City or mod structure. If the needle spins or refuses to settle, you are either already very close or the target is out of range; move a few chunks and check again.
Practical Navigation Tips
Travel in the direction the needle indicates, checking it every so often rather than constantly. Combine it with a vanilla map or a coordinate display so you can record where you have been. If you are in the Nether or another dimension, the compass will not help you; return to the Overworld first. Keep a spare in a chest at your base in case you die far from home.