There is no tashahhud after Sujud as-Sahw because the Messenger of Allah (peace and blessings of Allah be upon him) did not do that, as is indicated by the sahih hadiths.

If you forgot to do sujud as-sahw, you can make it up if you are still in the mosque and a long time has not passed. If a long time has passed or you broke your wudu or you left the mosque, you do not need to do the sujud and your prayer is still valid.