Ideally, you should make it possible for three-5h of transit time in Johannesburg, so that you've enough time for you to catch your connecting flight. Transiting in Johannesburg is very straightforward; simply just Stick to the signals for ‘International Transit’. The flight from Johannesburg to Harare can take about 1h https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/