This is the easiest and most fuss free way of travelling. I chose to go with Metro Turizm, one of the biggest bus companies here in Turkey.
First I bought my ticket in a Metro booth/shop. You can find a lot of these scattered around Istanbul. Look out for the blue signboards.
Next, check the schedule of the bus shuttle to the main bus station. The main bus station (otogar) is located very far away from the city center, and so it’s better to make use of the free shuttle that Metro provides. You should generally be at the Metro booth 2 hours before your time of departure. It takes about 40-60 minutes to get to the otogar from whereever you are in the city center.
When you get off the shuttle and into the otogar, there should be a Metro personel guiding you. If there isn’t head straight to the Metro counter and show them your ticket. They will usually direct you to your bus stand.
Once your bus arrives, the attendent will check you luggage in and put tags on your luggage. Make sure you take the slip with your luggage ID on it so that you can claim your bag when you reach you destination.
I personally like to tell the bus attendant where I am going, and tell him to remind me where to get off. They usually do.
And that’s about it! Travelling with overnight busses in Turkey is REALLY fuss free. Usually there is wifi and a USB port for you to charge your phone.
There is also small screens like in airplanes for TV but everything’s in Turkish!