S/s Arches And 'sills'

Alreeet

Just purchased some of those lovely looking s/s wheel arches with the s/s sills you can buy with them.... ive read posts bout how to put the arches on
making marks through the holes onto the existing plastic arches and then drilling through and screwing them on...
but i cant find anywhere how you put the 'sill' on...
its obvious it doesnt fit over the top of the rubber 'sill' you get as standard under the doors but they dont fit correctly onto the metal itself...
so how do we/i get em on? lol screw them on? or am i going at it completely the wrong way about it?
hope someone can help

cheers