Just install this extension the usual way like any other TYPO3 extension.
For Free Version¶
Via Composer using Command Line
composer req nitsan/nitsan-maintenance
Via Extensions Module
In the TYPO3 backend you can use the extension manager (EM).
Step 1. Switch to the module “Extension Manager”.
Step 2. Get the extension
Step 3. Get it from the Extension Manager: Press the “Retrieve/Update” button and search for the extension key nitsan_maintenance and import the extension from the repository.
Step 4. Get it from typo3.org: You can always get the current version from https://extensions.typo3.org/extension/nitsan_maintenance by downloading either the t3x or zip version. Upload the file afterwards in the Extension Manager.
Activate the TypoScript¶
The extension ships some static TypoScript code which needs to be included.
Step 1. Switch to the root page of your site.
Step 2. Switch to the Template module and select Info/Modify.
Step 3. Click the link Edit the whole template record and switch to the tab Includes.
Step 4. Select NITSAN Maintenance at the field Include static (from extensions):
Step 5. Include NITSAN Maintenance at the last place.