To install Template Hub on your Site Collection of choice, you still need to stay in the App Catalog. The deployment will be started from within the Template Hub App settings in the App Catalog.
Open the App Catalog in you SharePoint environment. If you don't know the URL, follow these steps:
Now navigate to "Site Contents", here you should see the app "TemplateHub". When moving with the mouse over the icon "TemplateHub" you should see a button "..." gets available, click on it:
Now choose "DEPLOYMENT" which redirects you to the settings page, where you can configure on which site collections Template Hub app will be installed:
You may choose one of it or a combination of them:
If you now open a site collection which match the criteria, the app was automatically installed and will be listed when you navigate to "Site Contents".