Open Word and tap the ‘Insert’ tab. Tap ‘Add-ins’:
Tap ‘See All’. Scroll through the list (there is no search facility) until you find PerfectIt. Tap 'Add' and tap ‘Trust it’ to install the add-in.
To access the add-in once installed, tap ‘Add-ins’ in the ‘Insert’ tab and tap ‘My Add-ins’:
Tap the PerfectIt icon to start the add-in: