Instructions are given in the Jaguar repair manual. They run to about 3 pages. You need three special tools, plus the parts and the consensus is that experienced mechanics have about a 30% chance of success.
'93 XJS 4.0 - '05 X type Est 2.0D - '13 XF 2.2D Sportbrake
Before committing to replacement, are you sure its the clutch that's failed?
Low refrigerant gas will interrupt the circuit as will poor connections. If you pull the two connectors off and zap it with 12V you will see the magnet pulling the plate if its working. Sometimes the plate distance can be excessive and the magnet can't pull it.
1988 3.6 XJ-S coupe
'Remember that the value of your investment can plummet as well as fall'