For this you just need to add content attribute and font-family attribute to the required element via :before or :after wherever applicable.
For example: I wanted to attach an attachment icon after all the a element inside my post. So, first I need to search if such icon exists in fontawesome. Like in the case I found it here, i.e. fa fa-paperclip. Then I would right click the icon there, and go the ::before pseudo property to fetch out the content tag it is using, which in my case I found to be \f0c6. Then I would use that in my css like this:
.post a:after {
font-family: FontAwesome,
content: " \f0c6" /* I added a space before \ for better UI */
}