How to underline words in a text on Facebook or on an Instagram post

You’re very simple and direct terms how can I underline words in in a text message, on Instagram, or on Facebook to make it stand out. I have read lots and lots of information about how to do it but it all sounds like gibberish to me and when I try it it doesn’t work. I know there has to be a very easy way to do this. I currently have a 12 Pro max iPhone, and I know how to use it but I need some simple, simple verbiage to tell me how to underline words. Please help.