Well I have a 1999 S80 T6. Doubt it would be much different. I tried to look up specific info for your vehicle from my VADIS but couldn't find anything other than in the general spec chart which shows a torque of 30 Nm (= approx. 22 ft lbs) for the spark plugs.
You will just have to remove enough stuff to get at the plugs i.e. the cover over the plugs/coils. I suspect you will have to remove the air duct which goes from the turbo to the inlet manifold/intercooler whatever if it comes across the top of the engine like it does on the S80. Then you have to remove the coils from the head. On the S80 each coil is held in place with a hex head bolt/screw so they have to be removed first. Keep track or mark the coils and make sure to note how the coil 'supply' wires are routed.
You can then get at the plugs with a long socket. Just crackk them loose first and then be sure to vacuum out any debris which may have fallen or comes loose down the plug 'wells'. Don't want any crap falling into the cylinders which the plug is finally removed all the way out.
Then remove the plug completely. I would do one at a time to lessen the likelihood of crap falling into the plug wells/cylinder. It is a good idea to vacuum out the well after removing the plug completely in case anything is hanging loose down in the well.
Install and torque the new plugs being careful not to cross thread them.
Then install the other stuff in reverse order.
Goes without saying to check the plug gap. I think you might find the plug gap and torque spec'd in the owners manual. They are pretty good at giving out some info. I couldn't find the plug gap on VADIS. Just call a dealer and ask them. They should also be willing to tell you/confirm the plug torque.