This particular spoiler was purchased from Smart-Parts-Direct in the UK.
If you are in the USA, head on over to Smartmadness for S-Mann gear.
You can, of course, buy it directly from S-Mann or from S2smarts.

Prep Work

The only prep work required is having the spoiler sprayed to match your car. 

Fitting the spoiler

The S-mann spoiler bonds over the top of the smart original spoiler, it isn’t totally necessary to remove the original spoiler from the car but it does make the fitting process that little bit easier.

The spoiler comes in 2 parts, the lower carrier and the top finishing piece.
In the pack you get a pack of fixing screws, you can use these but I have
always preferred rivets so I decided to use those instead.

Place the lower carrier onto the original spoiler, ensure it is centred
and mark the holes with a pencil. 

Drill an appropriate sized hole on each mark depending if you are using rivets or just making a
pilot hole for the supplied screws. Fit your fixings in place to hold the carrier to the original spoiler. 

If you use rivets, use suitable washers on the back to stop the rivet pulling through
and damaging the plastic. I used 4mm rivets about 10mm in length.

The carrier is now fixed in place by 4 points. 

Just to add strength, I drilled another hole in the leading face and installed another rivet.
None of these fixings will be visible when the top section of the spoiler if fitted. 

Take your body kit adhesive. Tigerseal  is the easiest to get in the UK. 

Depending on what you get, you may have to remove the tin base with a can opener. 

Place the top piece in place and ensure it is centred. Using a pencil, mark
the curve where the new and old spoiler meet. Remove the top piece again. 

Use masking tape to follow the outside edge of the pencil line.
This will make it easier to remove excess adhesive.

Using the fine sandpaper, rough up the area where the new and old spoiler will touch.
Wipe down the arear you just sanded with a wet cloth, this will help the adhesive stick. 

Turn the S-Mann top section over and apply the adhesive. 

Flip the part over and place into position.  Press the spoiler firmly in place all the way around and use masking tape to hold it in place while the adhesive sets.  If you have any adhesive coming out from between the spoiler, use the wet cloth to clean up the excess. 

Make sure the spoiler is taped in place for at least 24 hours at room temperature before removing the masking tape.  Any adhesive can be carefully trimmed back with a sharp blade. 

Refit the spoiler to the car if you removed it for this job. 

Job done.

Important

As you can see from the pictures above, I did the bonding in doors.
No matter how careful you are, you will get bonding agent on the carpet (as I did).
It doesn't come out and anything strong enough to lift it will destroy the carpet.

Always put down a sacrificial towel, large piece of card or something that doesn't belong to you.