Here’s a clear reformulation of those instructions:
Follow these steps to properly install your digital certificate (.p12 file) on macOS:
Open Spotlight by pressing Command (⌘) + Space and type Keychain Access.
In the left sidebar, select "Login" under Default Keychains.
From the menu bar at the top, choose File > Import Items...
Locate and select the .p12 file you previously downloaded, then click "Open".
When prompted, enter the "password" you created when generating your certificate.
Click "OK" to confirm. Your digital certificate is now installed.
Now we have to “TRUST” the certificate. Go to “Certificates”.
Find the certificate with your name and double-click on it.
Click on "Trust" and change "When using this certificate:" to "Always Trust".
And close the window with the “Red Cross” to save the changes. The password requested is your computer's password.
Our digital certificate is now installed and ready to use. You can check, depending on your browser, if the certificate appears here.