lot of work

First, make sure your fuel tank is almost empty! You will have to get it off the car!
Then, make sure you have some gas-resistant fuel hose that match the orifices of the Walbro pump. Those from the stock assembly will not be re-usable, and regular hoses will not do the trick (the rubber/silicon will get eaten by the gas and sooner or later you will loose fuel pressure).
Lift the car;
Disconnect the contacts/electrical line going to the fuel pump, from the right side under the car. Work with care, those plastic sockets are exposed to a lot of dust/mud and they will rather broke than get off
For the dual exhaust cars, the middle and final exhaust (cat-back) must get off
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Think also for the stock... so some more work!
Unscrew the bolts holding the tank straps and assist the fuel tank with a height adjustable platform.
Take the fuel tank down and unscrew the cap of the tank. Again, be careful and use an appropriate tool.
Once the cap is off, you can access the fuel pump assembly.
Take great care when unclipping the two yellow fingers holding all the assembly together! Those usually broke and you end up searching for another fuel pump assembly!
Once the assembly opened, cut the old fuel hoses (you will not be able to re-use them, as I already told you: they are thermo-retractile) and install the Walbro pump.
Re-assemble the whole thing and go back-wards...
Do a check of the new install before putting everything back (use some gas and support the fuel tank in position, start the car and check the fuel pressure with a fuel pressure gauge-that connects on the fuel rail and has a loooong line, going all the way into the cabin. Rev the engine a little, just to see the fuel pressure being where it should be (mind what kind of FPR you have).
If all is clean, strap everything back and that`s it!
I am posting some pics, hope they help
Good luck and let us know how it works out!
P.S. unfortunately, the og 9-3/900 ng don`t have a visiting hole under the rear seat, like the 9-5
There is such and opening, but not useful for the fuel pump install). Since you are there, you might get a cutter and make the hole bigger, for future interventions 
Get ready to work a little
First, make sure your fuel tank is almost empty! You will have to get it off the car!
Then, make sure you have some gas-resistant fuel hose that match the orifices of the Walbro pump. Those from the stock assembly will not be re-usable, and regular hoses will not do the trick (the rubber/silicon will get eaten by the gas and sooner or later you will loose fuel pressure).
Lift the car;
Disconnect the contacts/electrical line going to the fuel pump, from the right side under the car. Work with care, those plastic sockets are exposed to a lot of dust/mud and they will rather broke than get off
For the dual exhaust cars, the middle and final exhaust (cat-back) must get off
Think also for the stock... so some more work!
Unscrew the bolts holding the tank straps and assist the fuel tank with a height adjustable platform.
Take the fuel tank down and unscrew the cap of the tank. Again, be careful and use an appropriate tool.
Once the cap is off, you can access the fuel pump assembly.
Take great care when unclipping the two yellow fingers holding all the assembly together! Those usually broke and you end up searching for another fuel pump assembly!
Once the assembly opened, cut the old fuel hoses (you will not be able to re-use them, as I already told you: they are thermo-retractile) and install the Walbro pump.
Re-assemble the whole thing and go back-wards...
Do a check of the new install before putting everything back (use some gas and support the fuel tank in position, start the car and check the fuel pressure with a fuel pressure gauge-that connects on the fuel rail and has a loooong line, going all the way into the cabin. Rev the engine a little, just to see the fuel pressure being where it should be (mind what kind of FPR you have).
If all is clean, strap everything back and that`s it!
I am posting some pics, hope they help
Good luck and let us know how it works out!
P.S. unfortunately, the og 9-3/900 ng don`t have a visiting hole under the rear seat, like the 9-5