There are different extensions in many weight rooms that you can use when doing the tricep push-down machine.  In this picture we used the rope extension, but there are several others. 

The exercise starts with your arms creating a 90 degree angle.  Keep your elbows stationary and close to your sides.

Push down the weight.  Do not move your body to either side.  The purpose of this machine is to isolate the triceps, so you should use all arms.  You may find it easier to balance if you put one foot slightly in front of another.