You don't need to pre-reserve anything honestly EXCEPT a time to visit D'Light for a guided English tour. I just checked the Korean page and all it says is "Shop on B1F, tax free service available for foreigners." No area or section to do any kind of purchase pre-reserve. For tax free - just take your passport and say "tax free" at check out and they'll handle it all. If you really want to read the official rules for tax free procedure it's here:
https://english.visitkorea.or.kr/enu/SHP/SH_ENG_1_1.jsp . As you're spending over 300,000W - it presumably will be one of the delayed refund options you see there (depends on which service the D'Light store uses)