Catch the blue Airport 100 Bus into the City Centre and follow the directions as above for the train.
The Airport 100 bus is fastest and most direct, but more expensive; so alternatively, especially if you want to buy a day ticket and explore or travel around the City, the Lothian 35 will get you nearby the Baha'i Centre if you know your way around Edinburgh, whilst a 1 km walk (or ride on the 35) to the nearby Ingliston Park and Ride will enable you to catch the 48 / X48 which will get you to the end of Princes Street. See the Lothian Buses website if you need timetables and route maps.
Type your starting postcode or place in (A), then click Get Directions.
With driving, the basic thing to note is that the Centre of Edinburgh is full of one-way traffic and also has tram-works in progress, and rush hour is between 08:00-09:30 and 16:00-18:00. Your main route will be to come down Broughton Street and turn off into Albany Street.
Parking exists -
Outside the front of the Centre on Albany Street. On Sundays and all days after 18:30, it is free; between 08:30-18:30 Mon-Sat there is a 1.60/hr charge (max 4 hours).
There are also 8 spaces at the back; please note, that during important public events these are reserved for priority parking such as the disabled and also for unloading. For this small Centre Car Park, drive down Albany Street, right down Albany Lane, turn right, until you see the grilled gate on your right. You may need to check it has been opened.