How do you replace the serpentine belt on a 1999 Chevrolet Express van?

the diagram should be under the hood next to your latch to raise it.

Usually there is a tension pulley that a 3/8 ratchet can be used to move it to loosed it up enough to take off the old one and then put on the new one. See if there any arrows to place it on moving in the right direction. BUT THERE IS A SQUARE HOLE IN THE TENSION PULLEY THAT USUALLY IS 3/8 IN. Use a 3/8 drive ratchet wrench inserted into this square hole to release the tension. \

if you don't have a chart of the belt orientation on the engine compartment , usually near the latch / front of car, carefully draw a sketch by first drawing circles for all the pulleys then draw arrows around the circles showing the location ( Over the top or under the pulley) of the serpentine belt.

Make sure you crank the tension pulley all the way to make it easier to work the belt on and off