It is fairly straightforward but very, very messy.
I found it best to do the refill outside and on lots of newspaper. (wear disposable gloves).
Most of the refill kits I bought came with detailed instructions on how to do it.
I stopped refilling years ago. Now have only Epson printers, compatibile cartridges "cheap as chips".
I believe the Dell 720 is a rebadged Lexmark Z23 but with modified cartridges (the Lexmark ones don't fit), and it has always been cheaper to buy a new printer rather than replacement ink. (The ink is where their profit comes from).