The first thing required is to set the "cryptum-library" as a library. To do that click on the “Add Library…” button that appears on the bottom right of the preference window, and choose the cryptum-library document from your files. You will now be able to access all its styles and Symbols from any other document!
All elements in cryptum are symbols that are either the original material design symbols (found in the cryptum library) or symbols that are inside the cryptum file (like the widget headings, side navigation etc).
If you find it difficult or complicated working with symbols you can detach the instance you just inserted and use it as normal layers in Sketch. To do so left click on the inserted symbol and select detach from symbol.