You'll obviously need the proper coolant on hand to add to your engine, so make sure you have enough of the recommended coolant.
Best to start with a cold engine, or at least one not hot enough that opening the radiator cap will cause hot water to escape under pressure and burn your face or arms.
Top up the overflow to the cold mark, remove the radiator cap, and top up the rad to just cover the heater core.
Ensure the truck is in park ( I always use the parking brake), and start the engine.
Open the cab heater to max, so water will flow pushing any air out of the cab heater core...and give it time to warm up, maybe 10-20 minutes.
Watch for air escaping, and keep an eye on the coolant level in the radiator, adding new coolant as the level drops.
It will drop if any trapped air is escaping, how much you have to add depends on how much air got into the system during the water pump replacement.
Don't rush the process, and don't walk away, keep an eye on it at all times; add coolant to the overflow if required so it doesn't go dry (but not too much yet).
Once your satisfied you have all the air out replace the radiator cap, leave it running, and as pressure builds watch the level in the overflow; it should go up at least a little bit. If you had too much in there it may even spill out the escape.
Shut the engine off, and make sure the overflow hasn't dropped too much. Let the engine cool. As it cools it may draw from the overflow, so once the engine is cold, make sure you top it off back to the cold mark.
I just did my water pump, the main seal was leaking. All i had to do was fill it up and get it warmed up with the heater on hot, a couple trips around the block should do it. Just keep topping it off for a few days to a week. If after 7-10 days you are still adding coolant and/or smell coolant it may have a leak but it will take a solid week for all traces of the coolant smell to dissipate fully.
Do everything enforcer stated above plus this step..
When at idle with the cap off and the engine slightly warmed where the thermostat starts opening, slowly rev the engine to 3000 rpm and hold for a couple seconds. Then very slowly bring it back down to idle. Do that a couple times and you will feel the heat start coming through the vents. It you don't slowly bring down the revs it will overflow on you. Make sure you have the controls set to heat.
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