This is documentation on the legacy, non-automated critical CSS power-up for Autoptimze. The new, fully automated "Autoptimize CriticalCSS.com Power-UP" is now available at the wordpress.org plugin repository. Unless you know what you are doing, you should use that plugin and not the one below.
On your WordPress admin go to "plugins" -> "add new" -> "upload plugin"
Select the downloaded zipfile from step 2 and upload it.
Click on "activate"
Go to "settings" -> "autoptimize"
Click on the new "Critical CSS"-tab
Copy/ paste your license key from your criticalcss.com account in the license key field and submit
Congrats, you now installed the critical css power-up, you can now start creating rules for critical CSS!
How to configure your critical CSS once the plugin is installed;
generate critical CSS for the most basic of pages (the one which has your basic theme look and feel)
activate "inline & defer CSS"-option in Autoptimize and paste the critical CSS from the previous step
generate cricital CSS for a your homepage
in the Autoptimize Critical CSS Power-Up create a new rule for conditional tag "is_front_page", adding the CSS from the previous step
add additional conditional tag-based rule as needed (i.e. if you have parts of your site with a different look and feel, e.g. for WooCommerce-pages)
if you have specific pages that have a different look & feel, generate the critical CSS for those and create path-based rules to add that CSS as exceptions to the previous conditional-tag based rules