Compress the spring to 11.81 inches long from top to bottom (this is for a Volvo 850, refer to your own manual to see your specs) and put it on your new strut.
I replaced mine with Koni Sports Adjustable Struts. I can tell you they're firm and comfortable, and being adjustable is just so much fun!
Put back the spring seat, the cross shaped retaining nut (to 50 ft/lbs, or just as tight as you can with your pipe wrench, you don't want this to fall off) Put in the strut bearing mount and the washer, then put on the strut retaining nut (buy a new one) and tighten to 50 ft/lbs with your torque wrench.