I use to use HDA before I got OPL. I used my HDA disc to install FMCB.
CRT, I know that you use the PS2's disc drive to rip games to the HDD. You'll need some way to get the FMCB install ISO on your HDD. Obviously you can't just install the ISO to the HDD like you normally do, but it's actually really easy.
The easier (to set up) way is to remove the PS2 HDD from the console and connect it to a computer. You can either connect it internally as a slave drive, or by USB via an external HDD enclosure. Then you use a program on the PC to dump images to the HDD. The guide that nightrnr linked tells you to use Winhiiip. I always used HDL Dump. You'll get the same results with either.
So in short: Remove the HDD from the PS2 and connect it to a PC. Use Winhiip or HDL Dump to install the ISO on the HDD. Reinstall the HDD in the PS2.
The other way you can transfer ISOs to the HDD is not as easy to set up, but in the end it's the easier way to do it.
Anyway, once you get the ISO installed on the HDD, just use HDA to boot it. You'll put the noobie package (contains install files and a few apps including OPL) on a USB stick. Press a few buttons and FMCB will be installed on a memory card.
Tips:
Note that you have to format the memory card when you do this, so don't use one with saves on it. I recommend using only an official Sony memory card. Once FMCB is installed you can still use the memory card for saves, so you don't have to worry about it tying up a card.
You'll see when you launch the installer that there's two types of installs you can do. There's many different revisions to the PS2 console, and there's install files that are different depending on what revision you have. You can do a universal install, which will make FMCB work on any revision console, but it takes up more room on the card. Or, you can do a compact install that'll make FMCB work on THAT console only. It takes up less room on the card though. So if you only plan to use FMCB on that one console, do the compact install so there's more room on the card for apps and saves.
FYI, if someone was installing FMCB for you, they would use the universal install. Once you get the FMCB memory card on your PS2, you could change it to a compact install so it takes up less room. Or, leave it as a universal install if you wanted to share it between multiple PS2s.
The noobie package has OPL in it, but probably not the latest version. If it isn't the latest version, just download the current version of OPL and replace those files in the noobie package.
And, as already mentioned, all the games you already have installed on the HDD should work perfectly fine with OPL.