Older Jaguars had 12V Positive ground electrical systems. Converting to Negative ground allows you to install modern electronics, including alternators, radios, electronic ignition, and electronic fuel pumps.

Preparation
Get a copy of  the electrical schematic for your car. In the event you run into any unexpected difficulty, it will come in handy.

Check through your records to see if you've ever replaced any original equipment with solid state replacements. 

Disconnect the battery
The last thing you want to do is get a surprise shock, or damage your electrical system.Take the battery out of the car before you touch anything else.

Conversion
Remove your radio. Your positive ground radio could be damaged if it's not disconnected. Some radios are easily switched from positive to negative ground, some really can't be changed. Best bet is to install a modern radio (which may be why you're doing this to begin with). Converting a radio to negative ground is beyond the scope of this article, you will need to consult a radio specialist if you want to do this.

If your car has an ammeter, reverse the wires on the meter. If the car has a voltmeter, this step is unnecesary. 

There's no need to reverse the wiring on the oil, fuel, or temp gauges.

I believe that all positive ground Jaguars had either mechanical tachometers, or tachometers driven by a separate tach generator. In either case, do not reverse the wiring to the tach. (If the tach has been converted to a later electronic unit at some point, then you will need to refer to an  instrument specialist to convert it to negative ground.)

Reverse the wires on your ignition coil.This is very important: if you skip this step, you will lose 50% of your spark energy.

Reverse the connections to the fuel pump, since it may have been replaced with a transistorized pump at some point.

If you've ever replaced any original equipment (such as the instrument voltage regulator) with transistorized components, you will have to figure out how to reverse the polarity of each of these components. 

Now, connect the battery with negative ground: attach the ground strap to the negative terminal, and the supply cable to the positive terminal.

Locate the voltage regulator. Remove the "F" wire. Now remove the "B" wire, taking care not to let it touch ground. Touch the ends of the wires together. There will be sparks, don't be alarmed. This operation polarized the generator, preparing it to operate with negative ground. Reconnect the wiring.

You should be fully converted at this point.

Check out
Start the car. It should start and run normally. If power seems weak, you may have forgotten to reverse the wiring on the coil.

Test the heater blower. If output is low, then the blower wheel may have been replaced at some point with a directional wheel. Reversing the wiring at the blower motor should restore output.

Check the windshield wipers and washers. If either fails to function correctly, reverse the wiring.