Namaz-e Hadya-e Mayyit means the salat that is done for sending its reward as gift to a deceased person.
This salat can be done at any time, as many times, and for as many deceased people as possible.
Namaz-e Hadya-e Mayyit can be said like the subh prayer: only the niyyat (intention) will change. However, it is better to do this namaz in the method taught by Imam Ja'far as-sadiq (a.s.) as follows:
1st Rak'at: After Sura-e-Fatiha (Al-Hamd), please recite Sura-e-Ikhlaas (Qul Ho-wallaho- Ahad) 2 times.
2nd Rak'at: After Sura-e-Fatiha (Al-Hamd), please recite Sura-e-Takathur 10 times.