If any of the necessary steps are missed or omitted or delayed due to forgetfulness for example: Sitting between the second and third rakaats of those salahs having three or four rakaats Stopping between the obligatory acts more than a few seconds Omitting the Qunoot in the witr salah after giving the first salutation to the right, make two prostrations just like the other prostrations and recite the tashshahud all the way upto salam. Then, say salam finish the salah.