I see that you use a 1.9.2 installation, so the links to the v2.x devdocs and answers there won't be of use to you, since Magento 1.x doesn't support .less out of the box yet.
The short answer is this; regardless of how you compile the .less file (local, serverside or clientside), it outputs a .css file -somewhere-.
To load that css file in your custom theme you can either add it via
app/design/frontend/[THEME]/default/layout/local.xml) like this:
But working with these XML layouts ad a WordPress developer can be tricky, I know. Another (not very best practice) way to add a css is to open the
[THEME]/default/template/page/html/head.phtml file and just add it there in the
<head> like you would in a WordPress theme.
So if you managed to add a custom css file to the theme, the next step is to compile it with .less. Personally I do this via a plugin for my code editor that compiles local .less files to a certain .css file and publishes that to the server.
You could also take a look at this for a quick solution to render the less files on the server, but as they say themselves it's not a very good idea.
TL;DR: The first thing you need to do is to include a regular custom .css file in your theme. Once that's done it's up to you to find the best workflow to compile .less to that .css file.