This is for 1k's and hackable 2k's only. Do not attempt this on unhackable 2k's and 3k's.
First you need to update to OFW 6.60 and then install CFW, here is how you can do it.
You can either update to 6.60 OFW by system update or you can manually download it yourself. If you do system update with the psp skip these first steps.(also skip if your already on 6.60 OFW)
1) Download and extract the 6.60 OFW and keep it where you can find it:6.60 OFW - Downloads - PSPMOD.com - PSP Forum, Hack PSP, PSV Hacks, PS Vita Hack, PSVita
2) Go to USB connection on the psp and pull up the drive that the psp is connected to on the computer.
3) Now open the PSP folder and then the GAME folder and then in that GAME folder make a new folder named UPDATE (all caps)
4) Put the 660 OFW EBOOT in that update folder and then exit USB mode.
5) Go to GAME in the psp XMB and run it.
Now to install CFW.
1) Download the CFW from this link: 6.60ME-1.6 (OFW Version) - 6.60ME-1.6 (OFW Version) - Downloads - PSPMOD.com - PSP Forum, Hack PSP, PSV Hacks, PS Vita Hack, PSVita Now Go to USB mode on the psp
and go to the GAME folder just like you did in above steps. (See steo 3 above if you don't know how to find the GAME folder)
3) Delete the UPDATE folder because you will no longer need it. (Only do this step if you did the above steps for installing OFW)
4) Place the ME_INSTALLER in the GAME folder.
5) Exit USB mode and go run the ME_INSALLER. Run that in the XMB, go to GAME and then MEMORY STICK.
EDIT: FIXED Links.