[Theme: Pinboard] Single Category on home page

First off let me say that I know that this question has been asked many times. I have seen answers that involve index.php, function.php, home.php and one other file I can’t remember. All I want to do is restrict my home page to one category of posts. I have other pages for my other categories. There are a lot of posts saying how simple this is but I haven’t got it working. In some cases the suggested code doesn’t exactly match with Pinboard’s version. I have caused myself major problems a few times trying to implement some of the suggestions. I do not know PHP although I did use to be a programmer many years ago so I should be able to follow instructions.

If someone could please tell me how to restrict my home page to category 12 while using the Pinboard theme I would really appreciate it. It seems that the check for the portfolio category gets in the way of implementing some of the suggestions. My page that is currently under test and in poor shape is https://roadsaway.com. Thanks for your help in advance…….Jackie