Login into your SharePoint Online, go to the Site where you want to connect with Secured Signing. Click the gear icon on the top right and select "Add an app".
On the Site contents page, click "SharePoint Store" from the left, then input "Secured Signing" search.
Click the "Secured Signing" app, click "ADD IT" on the app details page. Then click "Trust It" on the popup window.
Now SharePoint Online starts to install Secured Signing, the app is gray out and would turns into blue after completing installation. Go to "Documents", check if "Secured Signing" menu appears.
Next: learn how to use Secured Signing App