Hi Will,

Try as I may I can't really describe it any simpler but I'll try.
Play a C Major Chord with the Bass E string fretted at the third fret.
Start with a T (Thumb)on A string-3rd fret, then Index on G string open, Then the T on D-2, then M on B-1. Begin again but alternate the thumb to the E-3 for the alternating bass.

Experiment with this pattern on other chords in the Key of C.
I hope this pattern comes through OK. This isn't typical TAB but it may be clear enough to be a help.

Don


         "C Major"
E-----------3-------------------------Thumb-------------------
A-----------3----Thumb--------------------------Thumb---------
D-----------2--------------Thumb------------------------------
G--------------------Index-----------------Index--------------
B-----------1-------------------Middle---------------Middle---
E-------------------------------------------------------------