Do you want Form 4852 as a substitute 1099-R or as a substitute W-2? Since you mentioned a 1099-R I will assume that's what you want. The steps for adding a substitute W-2 are different.
Before you use Form 4852 you have to try to get a corrected 1099-R. One of the questions you have to answer on Form 4852 is what efforts you made to get a corrected 1099-R.
To enter your 1099-R go to Federal Taxes > Wages & Income > Retirement Plans and Social Security > IRA, 401(k), Pension Plan Withdrawals (1099-R). Enter the correct amounts, not the amounts that are on the 1099-R that you received.
After you enter your 1099-R continue through the questions. You will come to a screen that says "Do any of these situations apply to you?" On that screen, check the box that says "I need to file a substitute 1099-R." After a few unrelated questions TurboTax will lead you through completing Form 4852.
This answer has been verified for accuracy by an Intuit expert employee
