Ideally, you ought to allow 3-5h of transit time in Johannesburg, so that you have sufficient time for you to capture your connecting flight. Transiting in Johannesburg is rather straightforward; just follow the indicators for ‘Intercontinental Transit’. The flight from Johannesburg to Harare normally takes about 1h 30m. KAYAK’s flight https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/