OK. This is easy. The black ink cartridge is the easiest. Even if you dont feel confident, very easy. Buy refill ink from any retailer as long as it specifies lexmark as being compatible. Ink kits cost around £10 but cheaps as good as expensive. The price doesnt reflect quality , it reflects quantity of ink. In the refill pack there is a small hand drill and a syringe. Drill a hole in the top of your black cartridge in the central raised bit, fill the syringe with ink and inject through the hole that you made into the cartridge. You know its full because it runs out of the printhead! My cartridge has been filled 20+ times out of a £10 refill kit. In lexmark terms thats approx £250 worth of cartridges for £10!
Colour ones a bit more complicated. Needs the lid gently prised off of the cartridge so that it can be taped back on. The colours of each reservoir are obvious. Fill each syringe with the individual colour and inject onto the sponge until the sponge will soak up no more. Same for each colour.
Run the printers head cleaner a few times with your 2 new cartridges in an et voila! ALMOST free ink forever amen!