Can’t do the hoot method, trans lines come off the trans together and there is a thermostat in the valve body that won’t allow flow.
Drain and fill a few times is the simplest. I’d suggest replacing the internal filter as well though.
Pull the large hex bolt on the pan to drain it (the little bolt is for checking level). To refill, you have to do it through the side of the case through an npt plug. Fill it, get the trans up to 170*ish and pull the little bolt on the drain plug while truck is running. Let it drain till it stops. Then put the plug back in.
I’d suggest sticking with dexron ULV. The tes 668 is Allison spec for their transmissions, this 10L1000 is not theirs but got their stamp to use in our trucks and be called an Allison. A thicker fluid is not a good idea for this trans (unsure if that 668 is or not)