Things like replacing the horn, installing a concealed radar detection unit, replacing the driving light units all require you to remove the front clamshell.
When you look it up in the Lotus Elise Workshop Manual, things look pretty easy... However, Lotus did forget to mention some minor things and experience also creates hints on how you could do it even better..

I made a description on how to remove the clamshell, tried to make some pictures at the right time and scanned some stuff from the Lotus Elise Workshop Manual.
The only thing I can add to below description is: Take your time! If you're not an experienced mechanic, you could end up breaking things if you want to do it the fast way...

To start with, you'll need to lift the front of the car. I did it using a simple lift and putting boxes filled with office paper under the front of the car.
You'll need to lift the car at the location of the blue stickers at the underside, you can find the lifting points on below sketch. They're called A:

Now, you'll have to remove both front wheels and the front wheelarch liners. At the passenger side of the car, there is a relay and fuse box fitted to the wheelarch liner. On below pictures, you'll find the car sitting on the box with office paper and the relay- and fuse box which has been taken off the wheelarch liner. This is done on a LHD car, if you have a RHD car, you might have to mirror the picture!

There's this other thing that you should not forget to remove... Your spare wheel ;) The can... it needs to be removed from the other wheelarch liner...

When you've done all that, have a look at below picture to see how the wheelarch liners are fixed:

direction and 'jiggle it out' of the clamshell. You might need to move the liner in a circle direction to get it out of the lower fixing points.
Don't pull too hard, you could damage the wheelarch liner!

Remove the two M8 fixings securing the lower rear edge of the clamshell to the top of the sill.
Remove the single M8 screw securing each top rear corner of the clamshell to a bracket at the top of the 'A' post. (open the door and you'll see the screw)
Remove the four fixings in the bonnet gutter, one at each rear end, and one at each front corner. The rear pair are also used as adjustable mounts for the bonnet closing buffers and use M8 captive fixings in the screen frame butresses. The front pair use M8 captive fixings in the radiator mounting panel. Take note of the shim stack fitted at each of the fixing points.
It's a good idea to use some glue fixing these shim stacks at the points where they need to be. Just to prevent having the shims at the wrong place...
Remove the five screws securing the front lower edge of the clamshell to the front alloy undershield.
Remove the front grille from the clamshell.
Remove the front spats (grille and spats in pictures below:)

Disconnect the clamshell wiring harness at the passenger side of the front compartment (for LHD that is!).

Now you can go two ways... Either you remove the bonnet to make live easy when moving around with the clamshell (I did it that way) or you leave the bonnet to the clamshell, cover the radiator with a thick piece of cardboard or so and handle the clamshell extra carefully because the bonnet is still attached to it.
Downside of removing the bonnet is that you'll have to re-adjust it.. Mine got stuck the first time and it took ne hours to get it open and adjusted in the right way.
Downside of leaving it attached to the clamshell is the risk of the bonnet falling in open or closed direction thus damaging the clamshell itself.. You might overcome this by sticking some tape to it or so...

Now get some pile of blankets on the floor. Put them in front of a wall or so. They need to be separated at the width of the clamshell itself.

Carefully lift and slide the clamshell from the front of the car. Note any washers or shims fitted at any fixing point!
Turn the clamshell with the headlights facing the ceiling and the underside of the clamshell facing the wall. Carefully put the clamshell on both pile of blankets and stick another one between the clamshell and the wall.

When ready with whatever you wanted to do, refit the clamshell in reversed order.

Clamshell fixings in below illustration from the Lotus Elise workshop manual:

Parts of above text are taken from the Lotus Elise Workshop Manual, as are the illustrations...

Considerations while having the front Clamshell removed:
The towing strut, which is fixed in the crash structure behind the front grille is victim to stone chips and not well prepared for that... You might consider doing the same as I did, remove it, remove it's paint and respray it with  Tectyl Anti-Stone chipping as illustrated in below pictures:

It might have something to do with me, but when I push that little center in my steering wheel, my car starts sounding like a shot duck!
When the frontclam is removed, installing a new horn, either a single high powered one or a set of horns might be a bright idea!

And... well.. while working on all those other things you wanted to do, installing a concealed radar detector might also be an option. To install such a unit, you'll need to install an antenna which faces the road in front of you and which is not covered by (pieces of) metal. So, installation behind the grille (that is, the grille of a standard Elise) is not an option... Look at below picture where you might put. (ofcourse I only did this for the picture as I wouldn't be having such a device which is nearly illegal would you? ;)

Oh... Another Elise owner (who owns a MY97) told me that the rings around the headlight units were badly corroded on his car. I had a look, mine were not... but it didn't stop me from spraying the metal parts with this Tectyl Anti-Stone chipping stuff.

Pictures below: Radar detection unit antenna and an attempt to photograph the sprayed headlight units:

Good luck!

And... as soon as you publish... you'll get comments (which I like anyway because I want to make this information as comprehensive as possible...  Thanks Lutz!
So, basically the comment is "you don't really need to remove the wheelarch liners!"
If you just unscrew them and put your hand behind it to remove the screw in the sill front end you can leave them on.
One remark, you'll need to have at least two pair of hands to do this job. When removing the wheelarch liners you can do it like I did.. alone...