Ive filled mine in years past (after flushing) by simply adding coolant to the radiator with the t stat removed until the coolant level reaches that point. Generally you will see some air bubbles coming up, I will slowly add until no more bubbles and coolant is at tstat level then drop t stat in, housing, and upper hose, then finish filling and top off resevoir bottle. Then start the truck and run at high idle for a few minutes, and top off again. Its always a good idea to check the level after a short trip, may need to add a little more. I generally flush my truck like this every 2 years.