Step 1: My Solution
Begin by acquiring the right materials:
A Watercolour Brush pen (popular brands are the ZIG Watercolour brush)
A paper clip or similar
Piece of paper
Step 2: ASSEMBLE
Next you'll want to use the paper clip thing or whatever you can to get the black plug off. (using teeth also work)
Next, unscrew the ink pot then roll the piece of paper up into a funnel and place in the end with one hand, use the other hand to pour the ink into the handle keeping on eye on how quick it fills up. Remove funnel, put in bin.
Hold it tip down and shake for awhile (approximately 5-10 minutes) until the bristles turn black.
Overall the Pentel brush pen cost me £10 with 4 cartridges, you can pick up the watercolour brush pen and an ink pot for around £5, it doesn't smudge, it's waterproof, it's nice and black AND the ink in the pot is way more than the 4 cartridges combined so it lasts even longer!
Disclaimer - If you use this tutorial you have to refer to the pen as the 'Alex Wonder Brush'!