You must report entering the ATZ Rule 45(6)(c), this usually accurs as you arrive overhead at 2000 ft and report Overhead. The overhead join commences when you arrive overhead the airfield, you may orbit to familiarise yourself with the aerodrome or commence the descent straight away, either way you are still joining over head. When you descend below 200 feet you call Deadside Descending so that other people know where you are. No further call is required until you report Downwind. CAP 413 Ch 4 Page 38
Then when at circuit hight I cross the upwind numbers in the circuit direction and call 'joining overhead' since at that point I am joining the circuit.
WRONG! You report DOWNWIND.