mount iso in daemon tools,
copy eboot.bin from USRDIR to memorystick for example i:\Enc
copy the prx files you want (usually in folders called mod(ule) or kmod(ule) or modules )to be decrypted, too, to this folder
apparently you don't need to copy the prx files, that's at least what
i heard, you are advised to use the prx files from a different 1.50 game,
for ex. winning eleven.
overwrite all the files in your extracted iso (for example pro evolution soccer or the hustle) with their decrypted counterparts that are in i:\Dec
--> rebuild iso with umdgen or vcdromx36
--> start with umd emu 0.8c with the settings "copy load" and "use boot.bin" set to on.