Welcome!
The various advice of the thread you posted looks pretty much correct upon scanning over it.
A modern telephone line only needs two wires, green (TIP) and red (RING) in most cords. On a modular plug and jack those are the two center pins.
It can be somewhat of a challenge to make a good connection to old tinsel wires, even when freshly stripped, as used in the old cords. Often the byproducts of rubber deterioration have already oxidized the surface of the tinsel threads too, but I have often succeeded by wrapping two or three inches of bare 28-gauge stranded wire from a modular cord, or one with spade tips, tightly around the tinsel, and then sealing it with solder. Finally, shrink-tubing can be used to protect the "splice".
Inside the telephone the red wire connects to the L1 terminal, and the green to L2, but this assignment is historically not consistent and polarity in this case does not matter. The yellow conductor in the line cord is not used and may be stored or connected to an empty terminal (GND) on the little phenol connection plate. It was used for grounded ringing, when the signal came from either L1 or L2 and returned via Earth ground to the central office.
When connecting the cord end directly to a modular connection block, the wire colors should be reversed, green to red, and red to green, because a standard modular cords reverses polarity once again. But, as mentioned, polarity is not a critical aspect today for simple telephones like this. In my picture with modular connection box all three wires have been rescued, and the yellow wire was connected to the green line wire terminal in the box.
