front ones have a peg that fits into a locating hole behind the door, then you tighten them against each other (drivers to passenger)
rear, same principle but no peg, they are also attached to the front via a length of bar going front to rear.
this was how it work on my old car, and going off what coggie said it sounds similar for the leon.
my old car did get marked, but this might have been through excessive weight (brought some 12 14foot long 2by4's) or the load moving about, (christmas tree last winter).
i'm not bothering this time round, as i found i needed an assistant to
help fit them which wasn't practical.