How to add ad block in the upper left-hand corner of posts.

I’d like to add an ad block to the upper right-hand corner of my blog posts. I tried simply adding the HTML code to the posts.php file and the page.php files, but that didn’t work. I have a feeling that it’s more complicated than just pasting the HTML code somewhere. Is there a guide for the CSS challenged on how to do this? I’m sure it’s something I could learn, if there were someplace where I could go to do it. The site is at https://www.moneydummy.net.