From Sarahan, you take a bus to Jeori which is approximately less than an hour to reach to. As of the time of writing, the bus leaves Sarahan at around 9AM but better check with the locals about the updated departure time of the bus.
From Jeori, there are various buses going to Sangla. When we were there the bus arrived at around 1:30PM (again, better check with the locals once you arrive in Jeori).
The trip from Jeori to Sangla took around three hours and the fare is approximately less than a hundred rupees per person (as of the time of writing).
Have a Safe Trip!
A Memorial in Jeori for those who died making the Hindustan-Tibet Road