If Possible, you need to make it possible for 3-5h of transit time in Johannesburg, so that you've ample time to capture your connecting flight. Transiting in Johannesburg is quite simple; merely follow the signs for ‘Intercontinental Transit’. The flight from Johannesburg to Harare can take about 1h 30m. The https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/