The final meal before Tish'ah BeAb should be eaten sitting on a mat or the like, but one may still wear one's leather shoes. The reason for sitting low at this meal is not on account of mourning, but to show lowliness.
One should be careful not to have three or more males sit together for this meal, because we do not want there to be an obligation of Zimmun (invitation to say Birkath HaMazon). To do so would imply a permanent situation which is something we do not want.
Provided that one did not accept the fast verbally, if one wishes to eat after this meal, and it is still before sunset, one may do so. From sunset on, one may not eat anymore till after the fast. One may, however, still wash and anoint Bein Hashemashoth (after sunset, but not after the emergence of 3 stars).
There is a difference of opinion as to whether one may still wash (and anoint) if one already accepted the fast verbally. However, since the majority of opinions is to forbid it, one should be strict in this matter.
(See Shulhan 'Arukh with Rama, 552:7, 553:1. Kaf Hahayyim, 552, Oth 37, 553, Oth 1)
Tishah B'av, Tish'ah BeAb, 9th of Av, 9th of Ab, Seudat Mafseket, s'udas mafsekes,