My 91 Silverado has the radiator drain on the lower corner (drivers
side) of the radiator facing the engine.

If you cannot find a petcock drain, I would just take the lower
radiator hose off at the radiator and drain the coolant into a
container and then take it to a collection / recycling center.

Don't let any pets or animals get into any of the coolant, it's sweet
tasting so they are drawn to it, and it will likely kill them if they
drink it. You have to be careful to clean up any on the ground, or any
that collects in or on the body of the vehicle after you have drained
and re filled everything. I usually hose everything off good when I'm
done working.

I use the Dexcool orange coolant from Prestone. I usually flush every
five years, but at the same time I change the upper and lower radiator
hoses, and the thermostat.

I always just take off the lower radiator hose, drain the coolant,
refill with distilled water, put the hose back on, run the engine a
little while, drain the water, and repeat the process until the water
being drained looks reasonably clean. However, after draining the
coolant the first time, and before filling back up with water for the
first flush, I take the thermostat out & put the thermostat housing
back on without any thermostat.

I usually buy and extra thermostat housing gasket, I take the old
thermostat out, then I put the thermostat housing and new gasket back
on, without any thermostat. This allows the water pump to circulate
water through the system fairly fast, and does a good job of flushing
the system. Plus, I don't have to heat the engine up and wait for the
thermostat to open before I begin flushing.

When I'm done flushing, I put in a new thermostat and new thermostat
housing gasket, and new upper and lower radiator hoses and clamps. I
also inspect the heater core hoses and recovery tank and hose.

I fill back up with coolant using about a 50/50 to 60/40 coolant /
distilled water mix, then run the engine with the heater on to make
sure I'm getting heat and the coolant has made it to the heater core
and there are no air pockets in the system. After the coolant has
mixed I usually test it to make sure it has adequate boil over and
freeze protection. I also check all hoses and connections for leaks
after test driving.

John