Step 1: Old City-Kabataş tram

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 Utilize a Istanbulkart (Public Transportation Card) for each form of transportation on the way (except a taxi). Take the tram from the Old City, across the Galata Bridge, to the first stop over the bridge. As you hop off the Tram you will walk towards your left (diagonally) across the street to catch the Tunel.

Step 2: Tunel up to Şişhane Metro

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The Tunel takes you up the steep hill from the Golden Horn in about 5 minutes. Exit the Tunel, and follow the signs to the Metro (green line)(towards the right). The name of this metro is called Şişhane.  Note: If you are
starting from the other end of Taksim Square, BEGIN here: take the metro at the Taksim Station and get on the green line and get off at the 2nd stop which is Şişli- Mecidiyeköy.

Step 3-4: Take the Şişhane Metro (subway) line to ​Şişli- Mecidiyeköy and the Metrobus.​

From Şişhane, ride the metro only three stops in the direction of Haciosman to Şişli- Mecidiyeköy. Exit. Follow the signs to Mecidiyekoy and the Metrobus (the underground portion may be closed temporarily due to construction, so you may need to come up to street level. If so, there are signs to the Metrobus. The walk is about 5 minutes from the metro exit.

Almost There :-)Get off at Darülaceze Metrobus stop

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You will need to ride any metro bus in a westerly direction for three stops and get off at the Darülaceze-Perpa stop. Walk up the ramp to exit, turn right. (Don't forget to scan your card for your refund just beyond the exit.) At the stoplight you will take a quick left to walk along the main road, keeping the orange school building on your left. It is a short 5 minute walk from the metro bus to the church. The entry doorway to the elevators is in between the Carrefour Grocery entrance and exits. Come inside and go the the 4th floor. The door to Nations is on the right. Here is a Google Pin for the building. 
goo.gl/maps/5oT4ixZmoycepnkQ6
The physical address to show a taxi driver is :
​Nations Church 
​Gürsel Mahallesi
Darülaceze Cad. No 36
Ekşioğlu İş Merkez (name of the building)   
A blok Kat 4 No 6
Kağıthane-İstanbul
Building is the blue windowed building  & the entry way is facing the main street and between the doors of the Carrefour Super grocery store as shown in bottom picture.