In the middle of a word after E it is pronounced like the English X in FLEXIBLE.

In the very few words where X appears at the start e.g. xilófono, it is pronounced as an S. This is the same for the very few words it appears after I such as mixto.

However the word México can be spelt with an X or a J (Méjico). The J version is preferred in Spain, however both are pronounced as per the J version.

The same applies for names such as Xaxier/Javier, Ximénez/Jiménez.