Mincha is davened earlier, to leave sufficient time for the Seudah Hamafsekes. [A Chiyuv recites the usual Mishnayos prior to the last Kaddish.]
Tachanun is omitted from Mincha onward until after Tishah B’Av.
The Seudah Hamafsekes consists of bread dipped in ashes and cold hard-boiled eggs. One may drink water, tea, coffee and other non-alcoholic beverages, but should consume less than he usually would.
The Seudah Hamafsekes is eaten when sitting on a low chair. One may still wear leather shoes.
Shir Hamaalos is recited before Bentching, but a Mezuman is not made. Ideally, three adult men should avoid eating this meal together in the first place.
Before bentching, one should verbally stipulate that he might still eat or drink again before the fast begins. [If one neglected to do so, it is still permissible to eat and drink.]
After bentching, one may resume sitting on a regular chair until Tishah B’Av begins.
After Plag Hamincha, if one accepted (whether verbally or in thought) to begin the fast, one may no longer eat, and all the prohibitions of Tishah B’Av apply. Even so, leather shoes may still be worn.
