There are two methods for that:
1) Get a hard mod done.
This involves soldering of a modchip like Infinity onto ur ps2 via soldering.
Preferably to be applied by a pro as it might damage your ps2 if you are not comfortable with soldering Stuff.
2) Apply a soft mod.
This is done by installing various files on ur ps2 memory card and using it all the time your ps2 is powered up.
For the files to be loaded, you need a pre modded ps2 which can play a dvd with the req files already burnt on it.
You can transfer the files to the memory card from the dvd then.