In most cases, the withholding takes place when the shares vest and enough stock is sold to cover the taxes, and most importantly, your tax withheld should be relected in your W-2. whatever RSU you sold can be input into Sch D as normal stock sale. for more info, refer to: http://blog.wenxuecity.com/blogview.php?date=201004&postID=4785
and
https://us.etrade.com/flash/olink/rsplayer.html
M2C