Get some doublesided tape and stick it under anywhere that will come into contact with the inner door sills. Then stick two self tappers in the front inner wheel arch and two in the back inner wheel arch.
Not sure if that makes sense.
The reason we used double sided tape is that bonding it makes it permanent, whereas the tape can be replaced over and over if the sideskirts ever need to come off.
You can also drill into the sill from the underside of the skirt and stick a screw or 2 in there but I prefer to drill as little through metal as possible.
There should be slight indents in the sideskirts to indicate where a screw should go.