How to accept a Fast

1. Kavana
2. Instert this somewhere in Shmone Essre:
הריני בתענית יחיד מחר. יהי רצון שתהא תפילתי ביום תענתי מקובלת
3. Instant Ta'anit!

Note: Rama says it is better to instert this after the Amida (as not to make a hefsek), but I remember being told to say it during Sh"E at Gush. One one of my friends forgot to insert this in his tefilah, R. Mordechi told him not to fast.
Supernote: Mishna Brura says on this in the Rama, "It's better after the tefilah: Like in the beginning of the section when is says: in "shomea tefilah" or "after he finishes his tefilah, etc." On this he [Rama] says it is better to add it in after the tefilah but before yehe ratzon."

שולחן ערוך תקסב:ו