TimeMachine over network without TimeCapsule
You will need to share the backup drive with AFP (setup under Sharing in System Preferences).   

On your laptop you'll need to go into Terminal and type

sudo defaults write com.apple.systempreferences TMShowUnsupportedNetworkVolumes 1


This will allow your laptop to see the backup drive as a supported Time Machine compatible network drive.   Select this drive in Time Machine preferences and you should be good to go.