Ideally, you should let 3-5h of transit time in Johannesburg, so you have ample time and energy to capture your connecting flight. Transiting in Johannesburg is very straightforward; simply Keep to the signs for ‘International Transit’. The flight from Johannesburg to Harare requires about 1h 30m. In the following sections https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/