The procedure is approximately this (assuming the tank is drained of fuel already):
In the engine compartment release the two fuel lines from the tank.
Unplug the fuel sender
Remove the rubber firewall seal/grommet that the fuel lines go through
Disconnect the fuel filler pipe and vent
Jack the car up - as high as you can at the rear
Support the fuel tank from below
Carefully release the two straps that support the tank
Lower the tank down
The point to be very careful of is when you remove those straps - if there is any corrosion the odds are that the straps will take some of the tank with them and you'll end up with a perforated tank. To a degree this may be unavoidable but you can minimise the damage by being careful. Given the likelihood of this you'll probably have to reseal the tank regardless.
The less significant point but almost as important is the fuel sender and pickup - be extra careful when removing this as the studs on the tank can shear off very easily, you also need to treat the seal with kid gloves.