Always flush and disable any caching plugins you might be using, before installing a new plugin.
OMGF Pro is an add-on for OMGF and requires OMGF to run correctly. If you do not have OMGF currently installed, install it for free directly from the WordPress.org repository.
To install OMGF Pro and register and activate your license, follow this guide.
After installing OMGF (Pro), a minimal amount of configuration is required to make it run.
The default settings of OMGF (Pro) should suffice in most situations. Simply head on over to its settings screen ( Settings > Optimize Google Fonts) and click Save & Optimize at the bottom of the page.
For help tweaking each of OMGF (Pro)’s features, continue to the User Manual.