Idiots guide.

First remove the lock,say its the one to undo the rear seat ,there is a slide clip that holds the lock in,slide it across and the lock will come out,remove the bracket on the back/bottom of the lock(opposite end from were the key goes in)there will be a circlip under the bracket,remove it,put the key in and turn it full,pull on the key and the lock inner will come out from the barrel,look at the lock and you will notice that all the tumblers will be flush ,take the key out and the tumblers will appear,put in the key that you want to make fit and you will see that some of the tumblers are still sticking out,these need to be filed off flush,a better was is to run it up and down a bench grinder,put it back together.Job done.