Menachot 6 - Non-Kohen Cannot Do the Flour Service

If a non-Kohen performs the service of taking off a handful from a flour offering, this offering becomes invalid. The same is true if the service has been performed by a mourner, by an impure person even if he went to the mikvah and will be pure at night, by a kohen who lacks some of the priestly garments, by one who has not washed his hands and a feet, who is uncircumcised, is sitting, or is standing on his colleague's feet.

If one did the service with his left hand, it is invalid, but Ben Beteira says that he can re-do it with his right hand.

If, while he was taking a handful, a pebble, a grain of salt, or a particle of frankincense came up in his hand, making the handful incomplete, the service is invalid. The rule is: if he took more than a handful or less than a handful, the offering is invalid. For example, if flour was showing between his fingers (too much) or if he took it with his forefingers (too little), the offering is invalid.

Art: Egon Schiele - Self Portrait with Spread Fingers 1909

Don't understand a point? Ask MosesAI.org about it.