If you want juice that contains nicotine the easiest way to do it is get your nic in 2x the strength you want to end up with and mix it with your doubler in equal quantities. The VG/PG ratio of your nicotine needs to depend on what ratio you want to end up with and how much VG is in your doubler. Maybe your friend has a bit of extra nicotine he/she could also give you?
If you don't need nicotine just add an equal amount of VG to your doubler.