How do you fix your favorite pen when it runs out of ink?

Go to your local office supply store and purchase a refill. Take the pen with you.

What kind of pen is it? That changes how you replace the ink. For some pens, such as rollerballs, it’s better to buy a new one with fresh ink and a clean nib. If you like the casing of the pen, you can purchase a refill at office supply / stationary store. If it’s a fountain pen, that’s somewhat different (mine uses ink cartridges, for example. I wipe the nip clean and swap out the old cartridge for a new one.)

I dip the end into the inkwell. My favorite pen is one I made myself out of a pheasant tail feather. I have to trim it occasionally, and I sometimes have to make a new one.