1. Remove cat 2. Get a piece of pipe in the diameter that you want to use in my case this is 2.25" 3. If you are not going to be using the cat again you can cut them off your cat or you can just buy 2 new flanges that match yours. 4. Clean the flange up with a wire brush to get it ready for welding and position it on the end of you pipe square. I chose to use the one that bolts to the header and work my way back. 5. Tack it in several places about every inch or so, so the metal doesnâ€™t warp when you get it hot. 6. After you finish all your tacks go ahead and run a full bead all the way around your flange kind of like this 7. Now take what youâ€™ve got so far and hold it up to the collector on the header and line up your other flange. 8. Now tack it in place. Pull it back out and finish your tacks and the rest of your weld 9. Now thatâ€™s all done go ahead and bold it all back up and feel your extra horses.