if you're a salary incomer, sooner or later from Jan. 15, 2019, you guys can download all of deductible items from the Hometax(홈택스: https://www.hometax.go.kr/websquare/websquare.wq?w2xPath=/ui/pp/index_pp.xml&tmIdx=0&tm2lIdx=0112000000&tm3lIdx=0112090000) like credit card usage amount, medical expenses, and so on.
Once you get all of these evidence, you need to submit to your employer for the year-end settlement(연말정산), but also you need to submit the previous employer's tax receipts, family certificates(주민등록등본) if any changes in your dependent list during 2018FY.
For this evidence, you need to ask to issue a donate amount receipts if you donated some or contributions to church, temple or any religious organizations.
Once you submit all of it, your employer must issue you a tax receipts(연말정산 영수증) to you by the end of February.
Your employer need to file all of employees' tax receipts to the tax office by the 11th of March.
Usually if you didn't submit any by mid in Feb., your employer settle your income as a single person without any deductions and you can refile it by the end of May for correction without any penalty.
Any questions, please send me an email to firstname.lastname@example.org