The best way to open an ink cartridge so that it doesn't leak is as follows:

(this applies to cartridges that look like the one below, which is similar for Epson and most Canon ink cartridges, and some Lexmark/Dell)



First remove the tape that is sealing the breathing hole (shown above by the yellow tape)

Then remove the protective clip (without applying pressure to the sides of the cartridge). Protective clip is shown below.



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