Step 1: Getting Started
*A piece of chewing gum(start chewing it before getting the other tools ; I did, and that's why it's not pictured)
*EDIT* a small piece of kneaded eraser would actually be better than gum for this, but I made this assuming not everyone who reads this has tons of art supplies
*a used cartridge that fits your pen
*a liquid syringe(I got this from my dentist after a mouth surgery, but I suggest trying your local pharmacy; be sure it isn't one you use for putting things in your mouth or other people's mouth ! Once there's ink in there you probably aren't going to want to put it in your mouth)
*A bottle or jar of ink in whatever color you want(make sure it's okay for fountain pens , some aren't;you can get this from a local art store for maybe five bucks)
*Paper towels or tissue(not pictured, but you need it)
You'll also want to do it somewhere where it's easy to clean up and in clothes you won't mind getting stained should your hand slip. I ended up doing this in my bathroom in a t-shirt and jeans.
Step 2: Cleaning Out the Old Cartridge.
What you're going to do is take your syringe and fill it with water from the sink. Your ink cartridge should have a hole in it where the pen originally punctured it. Take the syringe and stick it in the hole, then "inject" as much water as you need to fill it up almost all the way.
To remove the water, and the diluted ink with it, simply turn the whole thing, syringe and all, upside down and pull out the water. You won't get all of it, and that's okay, but get as much as you can and put aside the cartridge to dry some.
You can pull apart the syringe to rinse away any ink and dry the parts. Excess water won't help you where you're going next.
Step 3: Injecting the Ink
Now you're going to fill the cartridge. Take the syringe and open your ink. You only need a little, so don't fill it up all the way and make sure there's no air in there. Stick it into the hole again.
This is the cool bit. Firmly grab both parts of the syringe and pull the plunger out as much as you can. What you're doing is sucking out as much of the air as possible with the syringe, making a vacuum in the cartridge. Now let go and watch as the ink readily flows into the cartridge with only a little push from you.
It's important that you do it this way. If you just tried injecting the ink with out pulling out the air, you would have had to fight that air to get the ink inside. Sure, you could probably do it, but isn't this easier?
Remove the syringe (carefully!) and rinse it and put it aside before going to the next step, because unless you plan on using this right away, you'll need a way to seal the cartridge to keep it from drying up or spilling...
Step 4: Sealing the Cartridge
Just kidding ^^ If you don't want to use gum for this step, you'll need to find something that will stick to the cartridge and not soak up the ink.
What you should do with the ink that you've been chewing all this time (and I bet you thought it was for concentration) is you're going to take enough of it to cover the top of the cartridge and stick it right over the hole!
Don't try to push it in the hole; when you need to remove it gum may get stuck it there, which is messy and unnecessary. All you want to do is cover it. Then put a little piece of tissue on top of the gum, to keep it from sticking to other things. It time it will dry and make a soft seal that you can pick off when you need the ink later, but for now you don't want it sticking to something else and getting pulled off.
Because as easily as you can get it to stick to the cartridge, it sticks even better on almost everything else.