The standard route is via Natitingou, the regional capital of the Atakora department and the nearest town with reliable hotels and a working transport hub. From Natitingou it is about 45 km southwest to Boukoumbé on a slow but paved road. Local taxis-brousse (shared minibuses) make the run several times a day; a private taxi or motorbike taxi is faster and not much more expensive.
From Cotonou the trip is a full two days by road, normally with an overnight in Parakou or Djougou. Most travellers combine the Somba villages with a trip to Pendjari National Park, 70 km north of Natitingou — the two destinations sit naturally on the same itinerary. Walking shoes and a small daypack are more useful than serious gear; you will not need a 4WD unless you are heading deep into one of the more remote arrondissements in the wet season.