How to refill Dell 720 Cartridges?

I have acquired a second-hand Dell 720 printer with empty cartridges, which I want to refill rather than replace, since I am not sure whether the printer is working properly anyway. I have a JetTec refill kit but it has no instructions for Dell cartridges. I do not know where to drill the holes to inject the new ink and getting it wrong would ruin the cartridge. Can anyone help?

I believe this printer is similar to the Lexmark Z615, but I can't find instructions for that one either.