Here's my method.
Buy the appropriate oil which is most likely going to be GL4 gear oil. I had the choices of GL5 or Napa brand multi oil. It is supposedly YELLOW METAL SAFE and this is what you want. Do not use GL5 in a GL4 specified transmission. I used 3 quarts and it held almost all of it.
My example is a 5 speed from a Z installed in a 620.
Jack up your vehicle from the drivers side. Please, use a jackstand.
Remove drain plug from bottom of transmission. You may want to or have to spray it with your favorite penetrating oil first. In this case the drain plug is removed with a 1/2" drive ratchet. You should probably place something under it like a drain pan.
Give it a few minutes to drain then clean the threads of the plug and plug hole before replacing plug. It needs to be tight but be careful not to strip it. Don't worry if a couple of threads still show. Just make sure it's good and tight.
Next you need to remove the fill plug. This is located at a 90° angle to the drain plug on the drivers side of the transmission. It is probably going to have a large square head. In my case I had to soak the plug with penetrating oil multiple times. A large adjustable wrench can be used. I used a 3/4" wrench and a 2 foot breaker bar to remove mine pushing both feet against the crossmember and cussing profusely. If this doesn't work, give it another coat of penetrating fluid and go drink a beer while the blood returns to your hands.
Now for the fun part! Some people have special pumps and devices for filling the transmission. I suggest getting creative.
My technique is get around 4'~ of small hose. I used fuel line. Place one end of the hose into the fill hole of the transmission. You should be able to put it in and push it towards the back of the trans a few inches to keep it in place. Run the hose over the exhaust, torsion bar, etc. so that it comes out above the drivers side tire. Remove any seals on the gear oil bottle and cut the tip off. Insert the tip into the hose snugly so it will not fall out. At this point you can squeeze the bottle for a day or two or let gravity do it's thing. Add fluid until it runs out of the fill hole. Replace the fill plug, don't strip it. You may want to use anti seize. Here's my setup-
I put an oil filter wrench under my door mirror. The bottle us suspended by a screwdriver through the handle, your oil might not have a handle. Poke a hole in the bottom of the bottle to help the flow.
Enjoy your freshly filled transmission!
Please share any methods here.