If Possible, you need to allow 3-5h of transit time in Johannesburg, so that you've enough time and energy to capture your connecting flight. Transiting in Johannesburg is rather simple; just follow the signals for ‘Intercontinental Transit’. The flight from Johannesburg to Harare will take about 1h 30m. Streets are https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/