I wrote my first bootstrap based website about a month ago. Here is how it typically goes. You look around the web for a bootstrap template you want to use. Some are free and some you have to pay for. Once you find one you like, you download it and paste your text and images into the template. I think this is how most developers use it.
Make sure you test the template to see that it does everything you want, before spending a bunch of time substituting in your own content.