Installing click to call feature is quite simple, but you might need to ask your developer to do that for you.
Find the closer.init section on your website. This is the one you have added installing Closer widget for the first time. It looks like this (orgId is unique for your company):
Add those lines to closer.init section on every page, where you want to have instant call button.
Now you can add your personalized button which you can create from your CMS. Just remember to add onclick="closer.openWidget('autocall') in the button parameters.
Here is an exmaple of how this button could look like:
<button onclick="closer.openWidget('autocall')">Open with autocalling</button>