Fitting bubble arches is one of the trickiest and time consuming jobs you can do.  I very much doubt that anyone finds it easy, or there are many short cuts.  In order to run bigger or wider wheels you have to fit bubble arches to a mk 1, here is a guide on how to do:

Rear

 

1. - Trial fit you bubble arch, hold it on with a 'self tapper' screw each side.  Mark around it and remove the arch.

2. - The next stage is to work out how much of the old outer arch to remove.  I did this in stages to make sure I didn't make a mistake.  Ideally you should remove the outer arch until it is just below the level with the top of the wheel well, mark this with a pen.

3. - Using a spot weld drill remove the old inner arch between the centre of the wheel well and the outer arch and remove the outer arch to the line you marked earlier.

4. - Re-fit the bubble arch to see it all lines up.  What you are looking for is there being space to fit a new inner arch between the top of the wheel well and the bubble arch, but you will also need to weld the new inner arch to the old (whats left of it) outer arch.

5. - Once your happy you will need to make a template out of some carboard (corn flakes packet did it for me).  Trial fit the bubble arch with the template and make sure it is spot on.

6. - Transfer the template onto some 22 guage steel and cut it out

This picture is from my first attempt, that failed!!!

7.  Trial fit the new inner arch and adjust to fit, I failed the first time and started again with a new templete and inner arch!

8.  Once you are happy tack weld the new inner arch to the old outer arch.

My new inner/old outer join

9. - The next stage is to tack weld the new inner to the top of the old inner arch.

I should have painted this with Zinc primer first, but was in a rush.  Don't follow my example.

10. - Once you are happy finish off by fully welding both areas and paint and seam seal the new inner/old outer welds.

11. - Remove all the paint from where the top of the bubble arch will be welded to the car.  I used a flexi grinding disk and then repainted the area with Zinc weld primer.

12. - Re-fit the bubble arch and tack weld the the top of the bubble arch to the car.

13. - Tack weld the new inner to the underside of the bubble arch, not a nice job!

14. - Finish off all welds.

15. - Remove enough paint from around the bubble arch to allow you to blend the new arch in.

16. - Fill and paint (not as quick and easy as it sounds).

This picture is after a few layers of filler.