As mentioned, according to Ashkenazic custom and that of some Sephardim, Nefilat Apayim is performed while sitting by lowering one’s head and leaning it on his forearm while slightly inclining the face to the right, so that one does not directly face the floor. She must cover her face with clothing or a sleeve. It is not sufficient for her to bury her head in her arm, since the arm and the face part of the same body, and the body cannot cover itself (SA 131:1; MB ad loc.). The main reason that the face must be covered is tzni’ut, like that of one who hides her face from God out of trepidation and shame. Be-di’avad, one who is wearing short sleeves and does not have a handkerchief may lean on her bare arm. If there is a table there, she rests her head and arm on it, and the table is considered the main cover for her face (Peninei Halakha: Prayer 21:4).
Nefilat Apayim is customarily performed only where a Torah scroll or even other printed sacred texts are present. Where there are no sacred texts, the Taĥanun prayer is recited while sitting, without falling on one’s arm. In Jerusalem, it is customary to perform Nefilat Apayim even in a place without sacred books, since the sanctity of the city serves as a substitute for books (Peninei Halakha: Prayer 21:4).
Where it is impossible for someone to perform Nefilat Apayim while sitting – for example, if there is no chair available or if someone is praying the Amida directly behind her and she cannot go elsewhere – she may perform it while standing (MB 131:10). If she is next to a wall, it is best that she leans her arm and head against the wall, in the manner of Nefilat Apayim, so that without the wall she would fall. In this way, she is considered to be partially sitting and bowing (Kaf Ha-ĥayim 38).