first you remove the old cat leaving a gap between the front muffler and the header pipe.

1. measure this gap and add 6" or so to the length
2. accurately measure the outside diameter of the front pipe
3. accurately measure the inside diameter of the header pipe
4. obtain from an autoparts store the appropriate length of pipe of good heavy gauge steel that has at one end a flare to fit over the outside DIA of the front muffler pipe and at the unflared end is suitable to fit inside the header pipe
5. also buy an o2 sensor bung at the auto parts store
6. buy an exhaust seal ring of suitable size to fit around the unflared end of the pipe you bought

(http://www.amazon.com/Volvo-Exhaust-Starla-Brand-1266118/dp/B00AFBNPKQ)

7. purchase a 2 or 3 hole exhaust flange depending on what is on the header end and slide it on the unflared pipe end
8. slide the seal ring on the unflared end of the pipe such that 1-2 inches of pipe extend beyond the seal ring and the exhaust flange behind it
9. carefully tack weld "multiple" times around the seal ring so it is affixed to the pipe on the inside end (header side)of the pipe (or cleanly weld a bead around 100%, your choice)
10. now drill a hole in the pipe roughly where the o2 sensor would be if the cat was there and weld the bung on the pipe over the hole
11. install the flared end of the pipe over the front muffler pipe and slide the 2" extentsion end into the header pipe

clamp it up and you're done

PS: if there is slight angle between the end of the header and the front end of the muffler pipe you can utilize the curved section of the cat pipe in back of the cat in you fabrication to help in straightening out the run of pipe you're building