As for the right time for installing the lighting kit, not sure there is one ? the kit is designed to be installed either during the build or as an upgrade for an already constructed wing.
This method worked great for me:
After deburring the edges of the W-1203 wing skin I decided to make the landing lighting cutout while the wing skin was on the bench. I Clecoed a few ribs onto the skin to hold the proper leading edge shape then used the template to mark the landing light hole ? but did not drill the holes for the mounting ribs or lens. (That was done after the wing bottom was riveted in place and the wing was flipped this helps insure proper alignment).
The ribs were temporally removed and the skin was flipped over so the lower or bottom cut was just past the edge of the bench then the cut was made (slowly) with a jig saw outfitted with a 24 teeth per inch metal blade and duct tape covering the bottom as a pad to prevent scratches. The cut was also extended part of the way up the sides towards the leading edge but stopped well short of the tight bend. The skin was then flipped over and the ribs were Clecoed back in place to keep the skin tensioned, which prevents it from flapping around, and the cut was finished. A Dremel tool with a sanding drum was used after a little hand filing to smooth the edges. Turned out great and created no mess inside the wing to try to clean up.