The mod's content is gated behind the Deep Dark, so your first real task is locating an Ancient City. Everything else follows from there.
Locating the Deep Dark
Dig down to deepslate levels and look for sculk, sculk veins, and the telltale dark patches that mark a Deep Dark biome. Ancient Cities generate inside it. The most reliable method is to use the 
Ancient Compass, an item added by this mod that points you toward the nearest Ancient City. Craft it before you descend so you are not wandering blind.
Navigating the City
Inside, avoid triggering sculk shriekers. Sneak past them, or break them from a distance if you can do so safely. The mod's structures, including its temple-like areas, generate within or adjacent to the city. Look for 
Gloomslate, 
Chiseled Gloomslate, and 
Blooming Sculk Stone, which mark mod-generated rooms rather than vanilla ones.
What to Bring
Bring blocks to pillar up, a light source that does not disturb sculk, and food. The Sculk Centipede, Sculk Leech, Sculk Snapper, and Shriek Worm all live here and will punish a careless approach. Once you have found the city, mark the coordinates so you can return.