Once in the operating theater, you will be positioned so that your surgeon can easily adjust the arthroscope to have a clear view of the inside of shoulder. The two most common patient positions for arthroscopic shoulder surgery are:

Beach chair position. This is a semi-seated position similar to sitting in a reclining chair.
Lateral decubitus siting. In this direction the patient lies on their side on an operating table.
all position has some slight primacy. doctor choose positions based on the procedure being performed.

Once you are sited, the surgical team will:

clean (shave) hair, if needed
Spread an antiseptic solution over shoulder to clean the skin.
Cover patient shoulder and arm with sterile drapes
Most likely place patient forearm in a holding device to ensure your arm stays still during the procedure
Procedure
patient surgeon may inject fluid into the shoulder to inflate the joint. This makes it easier to see all the structures of patient shoulder through the arthroscope.
Then, patient surgeon will make a small puncture in patient shoulder (about the size of a buttonhole) for the arthroscope. Fluid flows through the arthroscope to keep the view clear and control any bleeding. Images from the arthroscope are projected on the video screen, showing patient surgeon the inside of patient shoulder and any damage.

Once the procedure is done, your surgeon will close your incisions with either stitches or steric-strips and cover them with a large, soft bandage.