I used lumines to downgrade. its not difficult to do, although i do suggest reading through the guides provided above a few times. You can buy Lumines from off ebay, but dont expect to pay anything less than £10 because its in demand. but this said, its well worth it because homebrew is pretty cool stuff.
However dont expect it to be simple as buying the game and downloading stuff, theres a lot to learn, almost three years of history of the PSP homebrew community you have missed... so get reading!!
I suggest wikipedia for starters, it condenses the history quite well:
Wikipedia Psp Homebrew