Go to portal.office.com and sign in using your username and password provided.
On the first page you'll see a small box in the top right saying 'Install Office'. Click on this and select 'Office 365 Apps' (see below):
The downloader package will begin to install (visible in your browser window at the bottom), depending on your internet speeds this could take up to 10 minutes:
The below screen will also pop up on the web page.
Once the download has completed, open Finder, go to Downloads, and double-click Microsoft Office installer.pkg file (the name might vary slightly).
On the first installation screen, select Continue to begin the installation process.
Review and Agree to the software licence agreement.
Select Install on the next page, don't make any changes.
Enter your Mac login password (the username will be your own, not Mac User as shown in this screenshot), if prompted, and then click Install Software. (This is the password that you use to log in to your Mac.)
The software begins to install. Click Close when the installation is finished.
You'll then need to open up a Microsoft programme, any will do it doesn't matter (e.g Word).
The first screen will prompt you to authenticate your account, you'll need to sign in again with your Office 365 username and password (the same you signed in to portal.office.com with).
You're done! Your software licence will now be applied.
Licence does not allow editing error
There's a common Microsoft bug at the moment where users get a yellow banner across the toolbar in Excel or Word saying that their licence does not allow editing on a Mac.
This isn't true and isn't a licensing error. To resolve this, open up Word and in the top toolbar select 'Word' and then sign out.
Close Word down and reopen it again, then sign in with your Office 365 details. The issue will be resolved.