Filling cartridges is simple. I've been doing it since 1996 when I got a 1965 Sheaffer Lifetime Pen finally cleaned out and started using it at work. Hated the idea of buying cartridges, the cost and waste (must be related to Nathan Tardif) and started using a syringe to refill them from bottles. Now I often prefer it to using converters. I recommend Goulet Pens ink syringe ( http://www.gouletpen.../GPC-InkSyringe ) along with the grease which is food grade 100% silicone (link in the post above). Brian also has a short video showing you how to fill a cartridge with the syringe.
The needles that come with these ink syringes are blunt. I refilled one Pilot cartridge for years so they can last a long time. Pilot cartridges have a large opening and are the easiest to clean out and refill. I usually use a polymer pipette to refill them. If you change ink you have to flush the old ink out of the converter. I fill the syringe with tap water and, while holding it over a sink carefully with the hole tipped downward, inject tap water into the cartridge repeatedly until the ink is cleaned out. It has been my experience that wet ink in a cartridge does not wet the poly material and flushes out very clean quickly. That is based on a couple of dozen bottled inks, most permanent waterproof ink.
I usually remove the syringe plunger and wipe it off with a paper towel, shake out the water inside and let it dry separated so there won't be any mineral buildup in the syringe from the tap water. Every month or two I will wipe the rubber tip with a bit of silicone to keep it lubricated and working smoothly in the syringe. You don't want it to get tight then loosen as you press the plunger, or you may squirt the ink harder than you intend.
About the only time I use new ink cartridges is when I want to try a new ink for a pen that comes in cartridges for that pen. This gives me extra used cartridges, a nice sample of ink to evaluate, and I'll buy the bottle when the cartridges get low. Now using Platinum Blue Black ink cartridges and I love this permanent modern iron gall ink. Will buy the bottle when I use up a few more cartridges.
Plus, the inks available for cartridges are so limited, life is too short to limit myself to inks in cartridges. So many of these cartridge inks are too ho-hum and mundane.