You will have to get a Pistol Grip Butt Stock, Trigger Guard Plate for the Marlin 1895SS or current 1895, and Curved Lever.
(1) Remove the straight butt stock by removing the 2 tang screws and pulling stock straight back. (Do not move stock up and down, straight back only).
(2) Remove the Squared Lever by opening and removing the one lever screw like you would if you were going to clean the gun.
(3) Remove the 3 screws that hold the straight Trigger Guard Plate in the receiver and remove the plate.
(4) Move the parts from the straight trigger guard plate over to the pistol grip trigger guard plate and reinstall in the receiver.
Be sure to hold the trigger to the rear to clear the hammer when reinstalling the trigger guard plate back in the receiver.
Do this process with the gun upside down or to it's side so the locking bolt will not fall out of the receiver. (If it does no problem, just drop it back into position) before reassembly.
(5) Install new pistol grip butt stock.
Check
http://www.e-gunparts.com/model.asp?idDept=158 for parts or get them from Marlin.
You may have to remove some wood to get the 1895 fore end to fit as the octagon barrel might be a bit bigger than the standard 22" barrel.