Install: Captisium SharePoint Add-In
A SharePoint Administrator will need to create a SharePoint App Catalog site if your SharePoint environment doesn't already have one. There is a great Microsoft document about how to do that here.
Download the Captisium Universal File Handler Classic Edition - SharePoint Add-In. This is a zip file. You don't need to expand the zip file, you can install it directly into your SharePoint Catalog.
Upload (or drag and drop) the Captisium Universal File Handler.zip file to your SharePoint Online App Catalog.
You then need to visit each of the SharePoint Online site collections that use classic libraries and add the app using the following method.
Select Site Contents
Click "add an app"
Click "From your organization" on the left-hand menu
Click the "Captisium Universal File Handler Classic Edition" App
Once the app displays the "Do you trust classic.universal.captisium.com?" dialog, click the "Trust It" button.
The Captisium Universal File Handler Classic Edition SharePoint add-in will be installed to the SharePoint Online Site Collection and is ready to be used.
Repeat the above steps for each site collection that uses classic libraries.
Once you have installed the SharePoint Add-In, you can test it by clicking the Add-In, and it will display your Classic Consent Process and the steps you need to do. Once you have completed the Classic Consent process, which must be done after the Modern Consent process, the screen will look something like this.