(i) The t-shirt lays on the back
(ii) The left hand takes the point (2) while the right hand grabs (3)
(iii) The right hand subtly goes to (1) and catches it (without releasing (3))
(iv) The apotheosis : both arms unfold themselves, and gravity acts
(v) Then both arms lay the t-shirt by moving suddenly backwards
(vi) Finally, a graceful move forwards for the last fold and you did it !
Remark 1 - left-handed ? adapt !
Remark 2 - moreover, you can decide the size of the folded t-shirt by varying (1), (2), and (3).