Best way to learn any tool from scratch is to Install and start playing around with it. I will suggest you to Install and start using and refer the below document and keep posting your queries in the forum.
Install and start referring User and Admin guides per KKT, it's a good idea,
Also you said you are already working on Essbase/Planning, try to get the metadata extract and start building the same onto DRM, a good idea is to start with just the PARENT|CHILD|DESC, once you get a feel of DRM Hierarchies, you can refer the following Tutorials to deep dive into DRM,
Hope you are having no issues , do update us the status or request you to close the thread.
Do post any queries if you have.