First you need to Download the Official launcher and the Mineshafter
Open the official launcher and Go to the Development tab you
Do Not Need to Log in Let it finish downloading
Then close the Official Launcher and open Mineshafter and Go to the Development tab
You Must Log in Then let it finish downloading Do not worry If you get errors
Then close Mineshafter and Open the official Launcher
You should be logged in Hit Play Offline Go to the Development tab and wait for it to finish downloading Minecraft should start.
Close the Official Launcher and delete it Now the Mineshafter should be able to run forge
Use this video after following my steps to learn how to install forge
https://www.youtube.com/watch?v=wHN_YomZbcs