Re-entry permit related

[haveQs]

My parents just got the interview call for green card (immigration based). They are here in the US and when they go in for the interview, they would like to apply for the re-entry permit (which allows them to stay in India for 2 years at a stretch). Can any one tell me the procedure for doing that? What are the forms that I need to fill?

Thanks

[abu lafya]

Not legal advise:
I think it is I-131, I think it is well self- explained.