travel from sha tin to lo wu in HK

Hi, i am looking at getting the day pass and i need to travel from sha tin to lo wu.
understand that the day pass does not cover lo wu station so how do i pay for the mtr cost to lo wu?

The day pass will cover from sha tin to sheung shui right and i will need to pay for sheung shui to lo wu?
At which station do i buy the ticket from sheung shui to lo wu?

Do i need to board another train at sheung shui since from this station, the train can go either lo wu or lok ma chau.

Need advice. Thank you!