How do I properly check and refill my automatic transmission fluid?

Hi! New taco owner and first time vehicle owner here and I would like to know how to properly check and refill my automatic transmission fluid. I believe the truck has to be running at operating temperature when checking and filling but correct me if I am wrong or missing something. When I check my transmission fluid while the truck is warm and in park the fluid barely reaches the end of the dipstick; when the engine is shut off and checked the fluid is nearing the "cold" mark. Any advice and assistance would be much appreciated!

 

The dip stick is on the passenger side of the engine bay. It should have two sets of marks on it, one for warm, one for cold. Best to warm it up to operating temp and run it through all the gears, (R, P, D, 2, L) then with it still running pull the dip stick, clean and recheck (like you would the engine oil). Don't overfill it.