Kuala Belait is 100 km southwest of Bandar Seri Begawan along the coastal highway. The drive takes around 90 minutes on good roads.
Bus services run between BSB's main bus terminal and Kuala Belait several times daily; the journey takes around two hours and is very cheap. Buses also connect to the Sarawak border crossing at Sungai Tujuh/Miri — you will typically need to change in Seria, 10 km up the road from KB.
For those continuing to Miri in Malaysia, the border crossing at Sungai Tujuh is around 20 km south of Kuala Belait. Taxis from KB to the border are the easiest option.