The best way that I have found for myself is to re-create magazine layouts, print ads, newsletters, newspaper pages, book covers and anything else that is off-line media and the printed page into an HTML and CSS version.
Doing so will provide you with several benefits, one is actually learning HTML and CSS much better and also giving you a collection of layouts that you can use as a starting point for a new site layout. Another important benefit is that you will be learning Pinegrow and any other applications as you get use to using them in real projects. During the process you will develop your own workflow.
This is of course just a suggestion and is something that not everyone will choose to do. Though I thought that I would share my own example of what or at least one of the ways that I have used to become better and learn more about designing and structuring individual webpages and entire sites. By taking a physical medium and re-creating it in code.