I used some 2" masking tape lined up to the bottom of the rubstrip and cut along the bottom edge of it, leaving a 50mm lip below the rubstrip.
I found that a stanley knife cut through the main part of the bumper, and a hacksaw at the very ends where the bumper is thicker. I used a dremel 'cos I have one! lol
take off the masking tape and sand along that line. I put masking tape over the rubstrip and the bit at the edges so I didn't get glue there.
then it's a case of applying sikaflex/tigerseal and holding it down with some masking tape there's a little tipy lip which fits up against the rubstrip. Screws in the ends and job's a good'un.
A little tip: once the glue is applied to the add-on, leave for 15 mins before sticking the 2 parts together so it's not so runny, it also sticks better initially.