Unbolt the shock absorber from either the bottom, or the top, it doesn't matter. I usually do it from inside the boot, behind the carpet. Then unbolt the anti roll bar links if you can. If you can't don't worry but it is a little easier if you can. Put spring clamps INSIDE spring, up through hole in the lower arm then clamp up as tight as you can. This way there is much more room. Then get a mate to stand on top of the disc / drum to lower the arm down as much as it will go, then with a lever bar, just pop the spring out.

Don't undo the bolts that mount the lower arm to the chassis as this will affect the geomety of the suspension and you will never get it right again.

I've done about half a dozen focus's this way and have never had a problem. New lowering spring should be shorter than standard spring so will go in the cups with ease.