https://www.timbertax.org/getstarted/sales/saletypes/sec631b/

https://www.law.cornell.edu/cfr/text/26/1.631-1

(c) Manner of making election. The election under section 631(a) must be made by the taxpayer in his income tax return for the taxable year for which the election is applicable, and such election cannot be made in an amended return for such year. The election in the return shall take the form of a computation under the provisions of section 631(a) and section 1231.