These busses leave Pasighat (next to the fish bazaar) around 11:00 am (ask for the real departure time).
The next morning around 06:00 you will arive in Guwahati (ISBT).
The bus fare is around 500 rupee.
Busses from Guwahati ISBT leave around 16:00 (4:00 pm) and will arrive in Pasighat next day around 11:00 am.
An alternative: take the ferry from Oiramghat to Dibrugarh and fly to Guwahati or take the train from Dibrugarg to Guwahati.
Travel agents that can advise/sell tickets are at same location where the Bus leave Pasighat (near fish-bazaar).