Blog page showing posts from single category

Hi there

I’m trying to do something that I think should be straightforward – display posts from a single category on my blog page.

I’ve tried a few suggestions I found by scouring the web but can’t make anything work. I’m not an absolute beginner but am no expert either!

Could anyone please advise on what coding I should use and which file I should amend.

I’m using the Twenty Twelve theme.

Many thanks

Kevin