Most likely, you report the short term capital gain or loss on form 8949 and Schedule D. Alternatively, you report it as business profit on Schedule C.
For more info, see a similar question at:
https://ttlc.intuit.com/questions/3399983-tax-issues-regarding-flipping-of-a-house
And references at:
https://www.hrblock.com/tax-center/income/real-estate/flipping-houses-taxes/
https://fitsmallbusiness.com/taxes-on-flipping-houses/ https://www.lendinghome.com/blog/how-to-maximize-house-flipping-tax-benefits/
