Once you have received the package installation URL from the Groundwork Apps team, you should install the package by visiting the URL and logging in using your username and password to begin installation. The installation URL will look https://login.salesforce.com/packaging/installPackage.apexp?p0=packageIdHere.
If you wish to install the package into a sandbox first to test, then you should update the package installation link to https://ltest.salesforce.com/packaging/installPackage.apexp?p0=packageIdHere and enter your login details for the sandbox environment.
You will then be presented with a screen like the one shown below.
Choose to install for all users as shown in the image and then press "Install". You will then be asked to approve the Messente api endpoint for third-party access as shown below.
Select the "Yes, grant access to these third-party web sites" and select "Continue". The application will then begin installation. After a short period the application will be installed and you will be presented the following confirmation screen.
Congratulations, you have successfully installed the application! You can now continue on and assign the correct permission sets to grant access to the app.