Summarize the following:
Most office chairs have a plastic tube over the extendable cylinder. Slide this all the way down or up, until you can see the metal cylinder underneath. You will not be able to adjust the height after this repair, so make sure you have it right. The seat of the chair should be level with your knees when you are standing.  If the chair won't stay up even when no one is on it, lay it on its side. If the plastic skirt covers the cylinder at this height, you will need to remove the skirt first. To do this, turn the chair upside down, push the retaining clip at the base with a screwdriver, and pull off the wheels, then the skirt. Slide the wheels back on. Get a ¾" (2 cm) hose clamp (Jubilee Clip) from a hardware store. Loosen the screw on the hose clamp (Jubilee Clip) and pull out the belt end. Wrap the clamp around the metal cylinder, but do not tighten it yet. The clamp will need to be very tight to hold the chair up. Give the clamp a better surface to grip by wrapping a strip of rubber or a couple layers of duct tape around the cylinder. Do this at the highest visible point on the cylinder,  Alternatively, scuff up this area of the cylinder with sandpaper. If the cylinder looks dirty or greasy, clean this off first. Slide the hose clamp to the top of the cylinder. Double check that the chair is at the correct height. Pull the hose clamp tight and fasten it by rotating the screw. The chair should now be unable to slide down past the clamp. The built-in height adjustment will still not work properly. If the chair is at the wrong height, move the clamp higher or lower on the cylinder. If the clamp slides off, fasten it over a strip of rubber to improve the grip, or try the PVC pipe method below.

Summary:
Slide the plastic skirt off the cylinder. Set the chair to the preferred height. Wrap a hose clamp around the cylinder. Improve the grip of the clamp (recommended). Tighten the clamp as far as possible. Test the chair.