If possible, you must make it possible for three-5h of transit time in Johannesburg, so that you have sufficient time to capture your connecting flight. Transiting in Johannesburg is quite straightforward; basically Stick to the signals for ‘Global Transit’. The flight from Johannesburg to Harare usually takes about 1h 30m. https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/