Walk up to the door.
Place your hand on the handle.
Depending on what kind of handle you have you will either pull down on the handle or twist it. On this handle pull down
Twist this handles clockwise or anticlockwise it doesn't matter
Push the door open while twisting or pulling on the handle. Don't push the door too hard or it will slam.
Step through to the other side of the door.
It is common curtesy to close the door behind you.