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It should state on the label whether the cartridge is compatible with your pen or not. You may be able to find cartridges at stationary shops, office supply stores, or online.  Monteverde, JetPens, Colorado Pen, and other brands carry cartridges compatible with Montblanc pens. You can use a standard fountain pen sized cartridge to fit a Montblanc cartridge pen. Hold the grip of the pen with one hand. Grasp the other end of the pen with your other hand. Turn the end cap counterclockwise while keeping the grip still. The end cap should eventually screw off. The cartridge is a thin tube inside of the pen. It should be pointing out of the nib and the grip. When you tug it, it will snap out of place. You can throw the empty cartridge away. The stopper of the cartridge (where the ink comes out) should go in first in the grip. Push the stopper into the grip right below the nib of the pen until it snaps into place. This snap means that the stopper has been punctured so that the ink can flow. The stopper will look like a clear, tapered end. Unlike the rest of the cartridge, which is filled with ink, the stopper will be clear. Insert the end cap of the pen back over the grip and cartridge. Turn the end cap clockwise to screw it back in. This will encourage the ink to start flowing. A few drops may come out onto the paper. Try writing a few words after a few seconds to help the ink flow.

Summary:
Buy ink cartridges made by Montblanc or a compatible brand. Unscrew the end cap of the pen from the grip. Pull out the old cartridge. Snap the new cartridge into place. Screw the end cap of the pen back on. Hold the nib downwards over a piece of paper for 5-10 seconds.