For Windows 8.1/10, right-click the Start button then select System.For Windows 7, click Start then right-click Computer and select Properties.Click OK and close the Computer Management window.Enter the account username and click OK.In the left pane, navigate to Local Users and Groups > Groups and then double click the Remote Desktop Users group.Click the Windows button to open the Start Menu, then search for and open Computer Management.If the user account being used to remote in does not have Administrator privileges on the computer, then the account will need to be added to the Remote Desktop Users computer group:.Place a check mark next to Remote Desktop under the Domain column and click OK to close the window.Click the Change Settings button and then scroll down in the list to Remote Desktop.Click the Windows button to open the Start Menu, then search for Firewall and open Allow a program through Windows Firewall.Other options are using Apple Remote Desktop (not very expensive from the App Store for unlimited clients, much less than 'retail' from Apple), instead of Screen Sharing (which is very basic, just VNC really) and using a VPN rather than SSH tunnelling.These steps need administrator privileges which may require you to contact your CTC or the IT Service Desk. Youll need to either forward ports on your router to access it over the internet, or use a VPN. Then use 'Connect to Server' in the Go menu of Finder and type: vnc://localhost:5901 ssh -L 5901:localhost:5900 5901 is the local port number (up to you, best to be above 1024 though).

This will work fine within the same network, however across the internet you will need to use SSH tunnelling to get Screen Sharing to work. Enable the built-in Screen Sharing, as well as Remote Login, in Sharing Preferences. I will cover using Screen Sharing remotely across the internet. Using Apple tools, rather than third party tools you have two options: Screen Sharing (free, inbuilt into OS X) or Apple Remote Desktop (inbuilt into OS X but admin client is paid for).

Mac remote assistance over internet