Sorry to hear that. I have replaced a few GM tanks, but not on the 2000-2005 Bonneville. I imagine the steps (for a backyard mechanic) would be lifting the rear ending, putting the rear tires on ramps, chocking the front tires, removing the negative cable on the battery, siphon out the old gas, pb blast the fuel tank straps, remove fuel pump connectors, remove gas tank straps (likely will be rotted and need to be replaced), then reverse order.
Sorry I don't have more to provide- a few additional thoughts is you likely will want to replace filler neck when replacing tank, they are very expensive but a used tank with filler neck from Texas or Arizona may be a option, buy new straps right away from RockAuto, dealer price on straps, ring, etc very pricy.
Good luck with the project.
2001 Bonneville SSEi, 219,000 miles.
Whats in my driveway:
1998 Chevy S10 4x4 ZR2, 1999 Olds Bravada, 2001 Pontiac Bonneville SSEi, 2007 Ford F-350 regular cab 2WD, 2008 BMW 335i coupe (Wife'* replacement for her Bonneville). The Bonneville is my favorite of all the cars in the driveway.