The process consists of creating, improving and transformation, requiring one Dream Matter and 245 Clusters of Solace at minimum. However, before you can use these, your world needs to have killed Gaz'haragoth, after which Eruaran will appear in Upper Roshamuul. He is the only one who knows what to do with these items.
The improving step depends on the item you bring. Unlike the first step, no dream matter is required here. Only your crude umbral piece and 75 Clusters of Solace are required to improve your item to umbral state. The chance of failing is higher here, and if so you will lose either your crude piece and keep your Clusters or keep your weapon but lose all the required Clusters.
The transformation step is the most valuable one and will take some effort, but the outcome is tremendous. Bring your umbral equipment piece and 150 Clusters of Solace in order to transform them to master state. If you are unlucky the following things may occur: you lose your umbral item and keep all Clusters or your umbral item is downgraded to a crude umbral item and you lose half of your Clusters (75).