How to implement. BEM to the project?

Question

BEM

I understand BEM technology, for me the question remains how to successfully integrate with a real project.
For example, there is a folder with a WordPress theme, I want to carry out the development process using the full BEM stack. To do this, I create a separate project next to the theme directory, then when the assembler builds the project, I automatically collect all the js and css using the same assembler and copy it into the theme.
How to be in this case with html, because the result of the collector is an html file, it turns out that I need to manually pull it apart into template files. This is not a big problem at the first stage, but if I make changes in the BEM project, then I need to pull this html into the theme files again. All this is somehow not convenient. How to be here?