My suggestion is don't. Refilling toner cartridges is a pain in the ass and just not worth it. It only takes one toner cartridge to leak and you're in for hours of cleaning (and you
will have one leak eventually.) Toner is
extremely difficult to clean without a toner vacuum, which uses expensive high grade HEPA filters and a special storage container.
Keep in mind that with most printers you not only have to buy toner, but you also have to buy a new chip to reset the page count on the cartridge.
It's just not worth the time, effort, mess, or potential damage to your printer.
I've found a reliable source for remanufactured toner from
Monoprice.
I haven't had any issues with their cartridges yet, and they're the cheapest around.