Which part of the privates does one have to shave?

In the name of Allah, the inspirer of truth

It is mentioned in Hashiya al-Tahtawi as follows:

And it is the sunna to shave below the navel with a blade, because it plucks from the root and is more strengthening. But in reality the sunna is fulfilled by anything that removes as the purpose will be achieved, which is cleansing. The word ‘shaving’ has come in the hadith, whether of the man or the woman. ‘Aa`na’ is defined as the hair which is above the private part and its surrounding. It is preferable to remove the hair of the buttocks out of fear of any impurity being soiled with the hair.

(Allama Tahtawi, Hashiya al-Tahtawi ala Maraqi al-Falah, pg 431)

And Allah knows best

Ilyas Patel