Tools Needed: 
Welder (!)
Grinder with cutting and grinding discs
Electric drill & bits
Safety goggles, ear muffs, welding helmet etc
13mm socket & spanner
Screwdriver
Lots of G clamps & molegrips                                       *more pics to be added soon*

Parts Needed: (depending on how bad yours is)
front valance panel
wheel well panel
2 front bumper mount panels 
2 wings front 
bumper

Time taken: An easy weekend

Panels - if you can get hold of the red painted bumper mount panels shown in the photos below 2/h unused at shows get them as they are far better quality & are original looking unlike the modern repro ones. You will need the bonnet left on and the set of wings you will be fitting, plus the front bumper handy so you get the fit right.

This how to assumes you need to replace the following: 2 bumper mounts, wheel well & front valance. If you only need to do a few of these, just pick out the relevant bits. Take you time, measure & trial fit as often as possible so that it fits right 1st time.

Getting started
Before doing any welding always disconnect the battery.
Take the front wings and bumper off including the bumper irons.
Next, for safety reasons, I usually remove the petrol tank. To do this undo the 4 13mm bolts holding the tank to the 'boot' floor. Now remove the breather pipes if there are any attached. Next unscrew the clamp holding the flexi pipe from the tank to the pipe that goes through the body. Bung the bottom of this pipe (a pencil or screwdriver is a handy size) and lift the tank out. Depending on if your washer bottle recess holding the brake fluid resevoir is being replaced, you might need to remove the resevoir (try not to get any fluid on the paint though).

Finally, you'll need to remove the bonnet catch from the front valance. This will either unbolt or you may have to drill out some rivetts. Don't bother to disconnect the wire pull.

The first cut
Before you attack it with a grinder, place the bumper mount panels over their originals and mark up how high they reach. Use this mark as a guide when gutting the old panel off. A good idea is cut at least an inch below the marked line so that you have some to play with. If you don't need to put the whole panel in, trim it down to size, then mark up on the car.  Once you've marked up cut through the sides and then across the bottom of the wheel well, just below the level of the spotwelds that hold the wheel well bottom panel to the upright bit

You can see the horizontal join in the pic of a panel here on the right.

Now that it's all cut off you will need to trim off the excess by this join using a spotweld cutter (it's a special drill bit) or careful use of the grinder. Once that's done, it should look like the one in the first picture below. If you ground it off, clean back the excess and drill out the spotweld holes - you will be able to see them in the metal.

Putting it all back on
Now, when you've double checked measurements etc, trim the body back to the line you made for the bumper mount panels to fit to. Next, clamp these in place.  Now do the same with the spare wheel well panel and finally the front valance. Does it all fit together ok? Play around with it 'till you're happy with the fit of it all.

Now, tack weld the bumper mount panels to the body and clamp the front panel on so that you can shut the bonnet. This will keep the wheel well in place. Check the fit by shutting the bonnet and bolting the front wings on with a few bolts. 

Happy with the fit? Now scribe or mark some lines on the panels so you know exactly where they fit together, remove the front valance and wheel well panels and seam weld the bumper mount panels to the body. Take your time and use a damp rag to avoid build up of heat.

Wheel well panel
Next to go on is the wheel well panel. Make sure the spotweld holes that you drilled out on the body are nice & clean and drill some more along the front of the wheel well panel (where it will attach to the front valance) and the sides (where it will sit on the ridge in the bumper mount panel. Line it up with the reference marks you made (the circular cut out in the middle of the panel is a good reference with the cut out on the body). Now fit the front bumper to make sure the bumper mount panel are at the right angle (once the wheel well is fitted you won't be able to move them).

If you're unsure, trial fit the front valance again. Now clamp the wheel well securely in position so that it's tight against the panels it will be welded to and plug weld it (see pic 2 above)

Front valance
Trial fit it again (pic 3) and bolt the wings on. Time consuming it may be but you need to make sure the bottom of the valance is exactly in line clamp it in place and weld it on. Some people spotweld it to the bumper mount panels, I prefer to seam it down the sides and then plug weld the bottom of the valance to the wheel well panel. N.B. the bottom of the front valance curves under the front of the wheel well panel.

Finishing it off
Grind down your welds so they look nice & tidy. Look behind where the wheel well joined the body and you'll see the panel curves around providing an excellent water trap. Fill this with seam sealer & paint over it to keep the water out

Bit of paint, put everything back together and go brew a cuppa