It is obligatory to follow the prescribed order of Wudu' starting with washing the face, two hands and elbows, wiping the head and two ears, and then washing the feet and heels, as related in the honorable Ayah (Al-Ma'idah 5: 6) and in accordance with what the Prophet (peace be upon him) did. Moreover, the Prophet (peace be upon him) said:
I will begin with that with which Allah began (i.e., mentioned first in the Qur'an).
As for Istinja', it is not part of Wudu', but it is a cleansing method for removing impurity that should be performed after urination or defecation and before performing Wudu'. However, if a long period intervenes between Istinja' and Wudu', a Muslim is not required to repeat Istinja' when performing Wudu'. Likewise, Istinja' is not prescribed in cases such as sleeping, passing wind, and the like.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family and Companions.