Step 1: 

Reach Torment Difficulty

To Diablo 4 Items start crafting Occult Gems, you first need to reach Torment difficulty. This is the highest difficulty level in Diablo IV, and it's where the Occult Gem recipes will start to drop. Once you're in Torment difficulty, head to the Headhunt zones, which are special areas where you can farm for these rare recipes.

Keep in mind that the recipes are random drops, so it may take some time to gather all the necessary ones.

Step 2: 

Acquire Occult Gem Recipes

Occult Gem recipes are critical for crafting. These recipes are a random drop from Headhunt zones, so farming these zones regularly will be your best bet. The recipes are what unlock the ability to craft specific tiers of Occult Gems (Magic, Rare, and Legendary).

Pro Tip: 

The drop rates for recipes can be low, so be patient and farm effectively to gather all three tiers of gem recipes.

Step 3: 

Talk to Gelena, the Coven Jeweler

Once you've obtained an Occult Gem recipe, head over to Gelena, the Coven Jeweler, located at the Tree of Whispers. Gelena will allow you to craft Occult Gems once you've acquired a recipe. However, you will not see the crafting option in her Occult Library until you've obtained at cheap D4 materials least one Occult Gem recipe.