Page 3 of you referenced article explains how to determine the reflected, and termination voltages of a transmission line from the incident source voltage. These are then used to generate the Thevenin or Norton equivalent circuits.
Do you know, in general, how to generate Thevenin or Norton equivalent circuits of a network? In not, Google should help.
Page 5. I don't understand how Vth value is chosen to minimize Ioh.
The equation Vth = Voh(min) - Ioh(max)*(Rd+Zo) appears to have fixed current at Ioh at maximum value.
How then is Vth minimizing this current?
About Figure B
I understand that the voltage at the end of the transmission line will be the incident voltage minus reflected voltage. So Vth = Vi-Vr
Also, after shorting the voltage source. Rth should be the parallel of Zo and Rt. Thus Rth = Rt//Zo