How do we offer tahiyatul wudu prayers, and do we perform it even in the prohibited times?

when can we pray nafal after wadu and how to pray it . Specifically i want to know can we pray wadu nafal on fajr and asar time as it is prohibited to pray nafal on fajr and asar time. And also is it necessary to pray tahiyatul wudu nafal while the body is still wet after wadu??