After purchasing from the App Store, Just Press Record should be available to install on your Apple Watch straight away. Apple Watch apps are installed using the 'Watch' app on your iPhone. If Just Press Record is not already in the section entitled ‘Installed on Apple Watch’, scroll down to the section entitled ‘Available Apps’. Tap the ‘Install’ button next to Just Press Record.
Occasionally, a newly downloaded App fails to appear in the list of apps that can be installed to your Apple Watch.
To resolve this, first please check that you are running the latest versions of iOS 11 and watchOS 4.
If you are up-to date, you should find that the app will be installable on your Apple Watch after restarting your iPhone and Apple Watch.