Ask HN: How can I improve my design documentation?

I am usually good at creating design and architecture for the problem at hand, but I really suck in documentation.

I get confused between different UML diagrams and how to put the architecture in different blocks and represent relationship between them? OR how much should this sequence diagram cover? Should it be covered in flow diagram or sequence diagram.

This is holding me back and I have no clue how to improve this. This has been brought up during the performance review.