According to Imam ash-Shafi^iyy, it is an obligation on one to perform ^Umrah once in one’s lifetime if one is able.
On the other hand, it is sunnah to perform ^Umrah often, especially in Ramadan. The one who performs Hajj may perform the ^Umrah immediately upon the completion of the Hajj. One has to go outside the boundary of Makkah’s Haram to make the intention to perform the ^Umrah.
For example, one may go to the area called at-Tan^im, where ^A’ishah Mosque is, and there say in one’s heart:
نَوَيْتُ العمرةَ وَأحْرَمْتُ بِها للهِ تعالى
“I intend ^Umrah, and I now become muhrim with it.” When one makes this intention, or a similar one, then the matters which were prohibited while one was in the state of ihram for Hajj become again prohibited.
Then one returns to Makkah, circles the Ka^bah seven (7) times to perform the tawaf, then one performs the sa^y. Then the man shaves his head or trims his hair and the woman trims her hair to complete the integral actions of ^Umrah. Performing the actions in this order is itself an integral of ^Umrah.