Navigate to the site with the cert you want to trust, and click through the usual warnings for untrusted certificates.
In the address bar, right click on the red warning triangle and "Not secure" message and, from the resulting menu, select "Certificate" to show the certificate.
In the window that pops up, select the "Details" tab (immediately to the right of "General"), and click on the "Copy to File..." at the bottom right of the tab.
This launches the Certificate Export Wizard; click "Next" at the bottom, which takes you to a radio-button dialogue for selecting the format. Leave the default "DER encoded binary X.509 (.CER)" and click next again.
Use the "Browse..." button to select a filename Documents (or wherever you'd like to leave the exported cert), and remember the name and path. Click "Next" to export the cert and then "Finish".
You should get another pop-up window telling you the export was successful. Click "OK" to dismiss it, and again in the original "Certificate" pop-up window to dismiss it too.
Next open the Chrome settings page, scroll to the bottom, and expand the "Advanced" section; in the "Privacy and security" panel, click on the "Manage certificates" area.
In the pop-up "Certificates" window, select the "Trusted Root Certification Authorities" tab, and click on the "Import..." button; this will launch the Certificate Import Wizard.
Click "Next" and, on the next page, select "Browse..." and use the explorer window to locate the certificate you exported at step 5 above.
Click "Next" again, then "Finish", and, in the "Security Warning" pop-up, click on "Yes"; you should see yet another pop-up letting you know that the import was successful.