Add simple share link in top of 2016

I don’t know why I struggle with this every time. I need to add a simple share link I already have to only the single.php file. When I copy and paste the template parts to a single.php file and upload it to my theme, it breaks my blog posts. I cannot find a step by step how to to do this correctly in the mess we call google. Please tell it to me like I’m five. I don’t want a template I have to tell it to use every time I make a blog post. I want to edit the default template for every blog post. Please help! ??