The details of refilling will vary from brand to brand and with the models within a brand.
As a generality, remember that laser toner is an EXTREMELY fine powder and blows away in the slightest draught. Be very careful about contamination of surfaces, particularly on and around the cartridges. Contamination between different colours, especially black, into the others can be devastating.
As a thought, only refill one colour at a time and give a day or so for the dust to settle before attempting another.