It's hard to integrate it into existing projects, cause Metronic isn't a UI library, Metronic is a UI theme, and it means need to use our Layout (nested HTML markup) and our styles.
We haven't published a skeleton guide for version 8. it's in development now and will be released in versions 8.1.1-8.1.2. Anyway. the guide will be the same as for v7 (just different folder/files paths).