The easiest method that I have come across is:
Remove magazine and ensure that weapon is unloaded and uncocked.
Depress the spring-loaded retaining pin located on the rear of the cocking handle and hold down while rotating cocking handle counterclockwise until comes off.
With the cocking handle removed, the pin and firing pin spring should come right out.
Turn the weapon barrel-up (in a safe direction) holding your hand under the rear of the bolt to catch the firing pin. (Replace magazine, trigger will not depress without it) Depress the trigger and the firing pin should drop into your hand. (Now remove magazine)
Next, grip the weapon in your right hand and depress the magazine catch button with your thumb. Press the muzzle against something (be careful not to mar the crown of the muzzle).
While the magazine catch and barrel are depressed, grasp the trigger guard with your left hand and pull straight down until it is removed.
The barrel assembly with bolt should easily slide forward for removal. Be careful not to lose the bolt retaining lug, which will fall loose during removal. If the weapon is reassembled without this part it would be dangerous to shoot.
To reassemble, reverse the procedure, of course.
My Uncle, who was a World War II Seabee showed me this method. In fact, he gave me a Type 14 Nagoya arsenal Nambu dated 15.6 (June 1940) that he took from a New Guinea battlefied.
I haven't tried to fire it because the firing pin is snapped off. My Uncle did this on-purpose, long ago, as he considered that it had been fired too much, already (Which was his opinion of all Nipponese arms).