This 2 raka'at namaz means that the salaat is performed for the sake of sending it’s reward, as a gift, to the Marhum. This salaat can be performed at any time, as many times and for as many Marhumeen as desired.
This salaat can be performed in the same manner as Subh prayer except that the Niyyat (Intention) is different.
However it is more recommended to perform this salaat in the method taught by Imam Ja’far-as-Sadiq (A.S.) as follows:
In the first unit, after Sura-Al-Hamd recite Sura-Al-Qadr
In the second unit, after Sura-Al-Hamd recite Sura-Al-Kawthar