Refer to the instructions for box 2a of Form 1099-R ("Generally, you must enter the taxable amount in box 2a."). To determine the taxable amount, refer to IRS Notice 2010-84 Q&A-7 and -9:
https://www.irs.gov/pub/irs-drop/n-10-84.pdf
and 26 USC 402A(c)(4):
https://www.law.cornell.edu/uscode/text/26/402A
