First. I'd buy a new OEM cable to do this that way you have your stock backup intact.
I finally got some decent pics. They're huge though, sorry about that. To start, completely uninstall the adjustable cable.
1st pic is the FIREWALL end of the OEM cable.
You can pull this apart. Pull the piece with the two hooks. You'll have to get the hooks over the lip of the cable but it's not very hard since they're pretty pliable.
Then you'll see the rubber piece that the first piece slid off of. Slide this rubber piece off of the plastic so that it's free.
Once both pieces have been freed up you have the pic below.
Now, you need a cutting wheel to cut the piece off in the pic below. You need to cut a slit big enough for the cable to slide out. AND while cutting into it make sure not to accidentally hit the cable itself. I've marked the pic below, you want to cut where I put the ugly White line. Once you have freed this piece from the cable the rubber piece will slide right off the end of the cable.
Once you have both pieces free of the cable you can discard them.
Since you already have the Steeda adjuster mounted in the firewall the next steps should be easy. Make sure the brass collar that cam with the firewall adjuster is in the adjuster. Push the firewall end of your cable into it and get it seated.
Now get under the dash and hook the end of the cable on the hook closest to the firewall on the quadrant.
Now get under the car and hook the other end of the cable to the shift fork.
At this point it's installed and you just need to adjust it. I adjusted mine so that I could push the clutch pedal down about 1.5 inches before feeling a good amount of resistence.
At that point I got in the car and drove around stopping once in a while to make a minor adjustment until I got it where I wanted it. Then tightened down the small set screw in the adjuster. Once I set mine it's stayed where I wanted it and worked perfectly. Going on 2 months now and one trip to the track with no issues whatsoever.