Preferably, it is best to enable 3-5h of transit time in Johannesburg, so that you've enough time for you to catch your connecting flight. Transiting in Johannesburg is rather simple; basically follow the indications for ‘Global Transit’. The flight from Johannesburg to Harare takes about 1h 30m. Countrywide Art Gallery: https://www.theluxurybooking.co.uk/flights-to-zimbabwe-from-london/