I have just been through this process myself. Will include the following photos at some point in my gallery section, but this may give you some ideas.
I made up the metal strips myself from some 0.8mm zincaneal sheet. At the early stage it was wider than needed and would be trimed to size when the flare was sitting on top. The pencil line indicates the outer edge of the lip
This was my guard when I started.
First thing is to get the lip nice and square to the body so the flare sits against the body without having to use glue.
Metal strip held in place by clamps.
Spot welded with the MIG. Only enough to hold in place in case I want to remove in the future. You can also use rivets.
Sit the flare on top and draw a pencil line to trim the strip.
Cut down with angle grinder and file edges to remove sharp edge.
Basic completed shape with welds.
Clips used for holding flare (is covered by chrome side moulding)
Flare should sit flat and without a gap against body.
Finished shot - took a day to do all 4 corners. After this stage I sealed the strips on the join with the lip with sikaflex so water cannot get underneath. John at Mini King also advised against using the black clips as they have to be screwed on and this in itself can lead to rust problems.
