1st very important thing; got all the hardware?
The Koni cartridge should come with a large bolt to go throught he bottom of the strut housing, in addition to a rubber sleeve to seal the joint between the insert and housing. There is also a plastic washer the goes over the Koni shock rod, and of course a larger diameter top nut to fit that larger rod.
What I did and recommend, is to go get yourself a pair of junkyard struts to do the hacking to, to be aable to do this at your leisure and not take your car out of commission. I think I found a pair on line for $75 or $100.
What you'll need to do is drill a small hole centered in the bottom to de-gas and drain the fluid. measure the Koni insert to just above the blisters on the body. That plus the bottom wall thickness is the length from the bottom of the donor strut to the cut point. It ends up close to the top of the donor. Hold the two side by side and you'll see. Either use a pipe cutter, or a hacksaw very carefully, making small cuts all the way around until the housing is scored, and the top and guts will come off and out. Then drill a hole centered in the bottom for the large bolt, sized appropriately.