is to plug in an ethernet cable to your main PC and the PC that you want internet on... obviously.
is to go to the normal control panel and not the stupid "metro" one or whatever it's called. (If you're on Windows 10)
Then use the search bar and type "Network Connections" It comes up as "View network connections" under "Network and Sharing Center"
I don't know about you but the actual "Network/Sharing" page does not have "Network Connections"
When you get there you need to right-click on "Ethernet", "Properties" and then "Sharing"
Select the option to "Allow other network users to connect through this computer's internet connection".
Now your PC can output it's internet connection to another PC.
Note: The host computer must be turned on (not in sleep mode) to output internet.