If possible, it is best to allow for 3-5h of transit time in Johannesburg, so that you've ample time to catch your connecting flight. Transiting in Johannesburg is very straightforward; just follow the signs for ‘Intercontinental Transit’. The flight from Johannesburg to Harare will take about 1h 30m. Off-peak Year: https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/