Okay! So if you tackle this song... I would do it like this (T = Thumb on bass string (5 for Am and 6 for G) D = Down strum (try with just index finger) U = up strum of bottom strings (also try with just index finger)) - Maybe even place your right little finger on the guitar body to keep your hand stuck in place...

T U D U / T U D U / T U D U / T U D U
Am /Am /G /Am