It is Sunnah for the worshiper in the prayers with the two tashahhuds to spread himself in the first tashahhud session, and the bed; He erects the right leg, brushes the left leg, and sits on it. As for the second tashahhud session, it is Sunnah for him to tawark, and tawark. It is to spread the right leg, and insert the left leg between the thigh and shin of the right leg.
As for the one-tashahhud prayer; as a two-rak'ah prayer; It is mustahabb for the worshiper to sit on the bed during the tashahhud session, and there is no harm in him sitting as he can if he is not able to sit on the bed. Because of foot pain, or enormity in the body, and so on.