How to theme a form with bootstrap
A good starting point is the bootstrap theme. They have a stable version for drupal 8. (It's the most object oriented theme I've seen so far)


  So I would suggest to make your custom theme, as sub-theme of it.


If you want to do it fully by yourself, you would use hook_form alter existing forms (if needed) and theme_hooks/html.twig to add some bootstrap specific classes and change HTML structure.
The mentioned theme is a good starting point to look at all the things that need to be changed.