If your fountain pen leaks at the nib then go to the first step.I bought a NVIZIO sunlight pen collection and when I filled it up I touched the back of the nib then I discovered the leak.
Step 1: Take Out the Ink
Turn the knob or on some pens take out the cartridge.
Step 2: Wash Out
Turn the knob clockwise and counter clockwise to suck in and push out water or soak it in water.
Step 3: Let It Dry
Let it dry
Step 4: Done!
Done! The extra ink that was left sealed it up then it didn't leak.