Capital gain on sale of Flats

STCG- A property was purchased in the FY 22-23 jointly by two individuals and ITR for that year was not filed (as there was no income) also no TDS at the time of purchase was deducted.....Now four flats are constructed on that property out of which two are sold in the FY 2023-24.

What would be the income tax compliances for the FY 23-24 and what will be the Cost of Acquisition for sale of the two flats for the purpose of Capital Gain?