I am trying to install a side skirts and a rear valance on my s13, wondering what material is better to use, 2-way tape from m3 or some type of silicone or glue will do?
I also used some screws to hold it to the body .
3M worked for my lip 
use the tape, glue may come out of the sides and look nasty and is more difficult to remove if you dont want the skirts anymore.
what kind of lip?? s13 zenki? if so I may have to this. some of my tie wraps snapped and the lip is the sag :?
doublesided tape, tie straps, when you make holes in the kit parts, make sure u heat them after u drill them so that you reinforce the circumfurence of the hole, otherwise the parts may crack
urethane window adhesive could also work
ziptie/ drill/ shoegoo
That’s what the professionals use. Black urethane window adhesive and double sided tape, plus a few screws in either end of the skirt (in the wheel wells). The double sided tape is really just there to hold it in place while the urethane dries properly; it’s waaay stronger than any screw or bolt will hold it in the long run. And it’s flexible to boot.