"It's the port forwarding I get stuck on....."
In principle it should be very straightforward. Login to the office router and configure a rule to forward TCP port 3389 to the office computer you want to access from home.
When you've done that you can check that you've set it up correctly by using an online port-scanning tool such as ShieldsUp! on the computer you want to connect to. Type the port number in the white box then click the "User Specified Custom Port Probe" button. The scan should reveal that port 3389 is "Open".
To connect from home, enter the public IP address (or host name if you have one) for your office into the Remote Desktop Connection window.