The seated tricep extension machine works the tricep muscle, which is the main muscle on the back of the upper arm. The tricep has three heads: the long head, the lateral head, and the medial head. The three heads work together to allow you to extend your forearm at the elbow joint.
First of all, adjust the seat so that you are comfortably able to sit and set an appropriate weight for you.
There are two different types of attachments available on this machine one higher one lower.
To start on the lower attachment, sit down with your back firmly against the back pad. Grab the handles of the attachment making sure there is no tension from the machine. You may choose to lean forward slightly so you do not catch the back of your head. Let your elbows do all of the work and push straight upwards. When you rest this excercise slowly lower back down until there is no tension and you can then hook it back onto its resting position.
For the higher attachment, sit down with your back firmly against the back pad. Grab the bars of the attachment pulling towards the top of your head leaning forward. This is more of a straight angled excercise where you will use your elbows to push the attachment forward in front of your head. You will find this one is a bit easier to slowly let go putting it back.