The purpose of the overhead join is to allow you to observe the airfield below and the signals square, it originates from non radio days. It is important that you cross the landing threshold before descending on the deadside, ideally cross the runway at 90 degrees and close the throttle fully, maintain your cruise speed to give you a reasonably high rate of descent, good visibility, and manoeuverability to avoid others, staying close to the runway so that you are visible to other traffic. Cross the upwind threshold at 90 degrees over the numbers, at circuit height, so that you have a good lookout into the circuit. This will keep you clear of departing traffic then fit into the downwind patern.
If you are familiar with the airfield you can join on the deadside descending but, if its a new procedure to you, start by flying onto the live side first and have a good look below before descending.
Remember some airfields do not have a useable Deadside, it may be used by gliders or helicopters.