How does an apostate come back to Islam?

If a person leaves Islam then decides to come back to
it, what he has to do is to bear witness that there is no god except
Allaah and that Muhammad is His slave and Messenger. If his apostasy
was because he denied one of the basic principles of the religion, then
his return to Islam cannot be complete until he affirms the principle
that he had denied. There is no particular time period within which
he can return to Islam; his repentance will be accepted until the point
when the death-rattle sounds in his throat and his soul departs. If
he is guided back to Islam within the time when it is possible (i.e.,
before death) and he performs as many of the Islamic duties as he can,
then he is a Muslim.