The following "How to..." will hopefully help you to fit the HiPacs without too many problems.
This is a guide only, you may wish to do things differently which is fine, all I can say is "don't rush - take your time".
The engine will have to be removed from the car and it would be easier with the rear wings off - although I managed fine with them fitted.
Wear the correct safety gear / eye protection, as you will be working in confined spaces.
Tools required for the job:
Paint Scraper; Thinners; Marker pen; Angle grinder (with cutting disc and sanding disc); Drill; 8mm drill bit; Clamps (mole grips / G clamps etc); Mig welder; Tin snips; Hand file (flat); Flat screwdriver; Hand file (round); Patience (a large helping).
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1 - Lift up the tabs which secure the sound deadening cards.

2 - Remove the cards.

3 - Depending on the year of your car the wiring will need re-routing.

4 - Temporary relocation of harness for "How to..." purposes only.
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5 - Remove underseal from inner wing in the general area to be worked in. A hot air gun is useful here but not essential.

6 - Then wash the area clean with thinners.

7 - Clean off inner wing inside engine bay.

8 - Then wash the area clean with thinners.
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9 - Position the flange using the side tray and firewall as a guide (arrowed). The flange can be easily manipulated at this stage to produce a snug fit against the inner wing. Mark off the area approximately 25mm inboard of the flange with the marker pen.

10 - Now cut out the area with the grinder.

11 - Remove the section. The excess material left in at this stage will stop distortion at the welding stage.

12 - Now clamp the flange back into position and mark through the screw holes with the marker pen.
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14 - Drill out the holes using the 8mm drill bit.

TIP! If you have access to a bench grinder and are confident enough to sharpen a drill bit, try re-profiling your drill bit to the shape in the picture. You can see the point in the centre touches the work area first and keeps the drill bit in position and then the outer edge of the flutes which are angled forwards begin to cut the outer edge of the hole until a small disc pops out. Similar to a wood drill bit. If sharpened correctly these produce nice clean burr free holes.

15 - Now fit the flange with the 9 screws ensuring each screw is rotating freely. File out any holes if necessary. Now mark around the inner and outer edges of the flange with the marker pen.
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16 - Remove flange and mark off for plug weld holes, approximately two between each screw hole.

17 - Drill out using the 8mm drill bit.

18 - Refit the flange using all the screws, this helps pull the flange into position and stops you from inadvertently welding up a screw hole by mistake!

19 - Now begin plug welding the holes up. You will need a high setting on your Mig for this. Practice on some scrap pieces first until you achieve good penetration and are confident to weld on the car.
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20 - You can see from this shot the orange glow on the flange meaning good penetration. Use clamps as necessary to keep the joint tight.

21 - When all the holes have been plugged, remove the screws and dress the welds down smooth.

22 - All welds dressed.

23 - Now remove the excess material that was left on, using tin snips/grinder etc.
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24 - Dress the inner wing down flush with the flange.

25 - Then finish off with the flat file removing any burrs.

26 - Finished shot from under rear wing.

27 - Finished shot from engine bay.
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28 - Stainless cover in place viewed from engine bay.

29 - Stainless cover in place viewed from under rear wing.

30 - With engine back in, access to carb nuts is easy using a modified spanner.

31 - Custom modified spanner.
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32 - Plug removal made easy.

33 - Plug leads easily removed and replaced.
Take your time fitting this kit and it will serve you well as you reap the benefits in the future. Bruised and scratched arms and cramped fingers become a distant memory. Initial fitting time is soon recouped after a couple of plug changes. 'Service with a smile' (On your face)!