IRS Publication 575 covers this.
Rollovers to Roth IRAs. You can roll over distributions directly from a qualified retirement plan (other than a designated Roth account) to a Roth IRA. You must include in your gross income distributions from a qualified retirement plan (other than a designated Roth account) that you would have had to include in income if you had not rolled them over into a Roth IRA. However, special rules apply for any amounts rolled over in 2010. See How to treat 2010 Roth IRA rollovers , later. You do not include in gross income any part of a distribution from a qualified retirement plan that is a return of contributions to the plan that were taxable to you when paid. In addition, the 10% tax on early distributions does not apply.
Any amount rolled over into a Roth IRA is subject to the same rules for converting a traditional IRA into a Roth IRA. For more information, see Converting From Any Traditional IRA Into a Roth IRA in chapter 1 of Publication 590.
How to report. Enter the total amount of the distribution before income tax or deductions were withheld on Form 1040, line 16a; Form 1040A, line 12a; or Form 1040NR, line 17a. This amount should be shown in box 1 of Form 1099-R. From this amount, subtract any contributions (usually shown in box 5 of Form 1099-R) that were taxable to you when made. Enter the remaining amount, even if zero, on Form 1040, line 16b; Form 1040A, line 12b; or Form 1040NR, line 17b.
The 1099R should come from the organization that had the original 401K, only they would know what would go in each box on the form 1099-R.