Jack it up and chalk wheels leaving yourself enough room to get underneath with a few socket extentions, I used a long bar for leverage. I sprayed penetrating fluid on first as the bolts looked pretty rusty. Be careful you dont round them off.
If you have a helping hand like I did, get them to hold the gasket in place whilst you fit the bolts to the new downpipe.
Once thats done its a doddle from there to fit centre pipe.
If you need more help Winny ill upload some photos of mine.