Human sacrifices to divine the future seem to have been made in the same sort of way as is commonly depicted in the sacrifice of a cockerel or rooster. Magical rites were performed, the animal was killed and then studied for meaningful signs.
The Celtic druids were fond practitioners of human sacrifices. They preferred to use criminals and slaves, but such was their need they would resort to sacrificing innocents if these were in short supply-
"They would strike a man devoted as an offering in his back with a sword, and divine from his convulsive throes. Without the Druids they never sacrifice."
"They deemed it indeed a duty to cover their altars with the blood of captives and to consult their deities through human entrails."
This may just be Tacitus' interpretation, as the Romans were fond of divination through animal entrails.
A further example comes from the exciting tales of explorers John Hanning Speke and James Augustus Grant while searching for the source of the river Nile. Speke needed an escort from the Ugandan king Mtesa through other territories, but Mtesa ‘almost daily’ executed anyone who failed to ingratiate themselves enough. Speke wins him over by delighting the king with the novelty of his courage in protesting the execution of one of the King’s wives. She had offended Mtesa by offering him a piece of fruit.
Upon gaining the escort Mtesa orders a sacrifice-
"Speke consequently informed the king that all he required was a large escort to accompany them through Usoga and Kidi to Gani, as further delay in communicating with Petherick might frustrate the chance of opening the Nile trade with Uganda. The king replied that he would assemble his officers, and consult them on the subject. He exhibited his folly, however, by allowing his people to make an inroad into Unyoro and carry off eighty cows belonging to Kamrasi. To their horror, Kyengo, the chief magician, informed them that the king, being anxious to pry into the future, had resolved to adopt a strong measure with that end in view. This was the sacrifice of a child. The ceremony, which it fell to the lot of Kyengo to perform, is almost too cruel to describe. The magician, having placed a large earthen pot full of water on the fire, arranges a platform on the top, and on this he binds a young child and a fowl, covering them with another pot, which he inverts over them. After the fire has burned for a given time the upper pot is removed. If both victims are dead, it is considered that war must be deferred for the present; but, if either should be alive, it may be commenced immediately. When the army is about to proceed to war, the magician flays the young child, and lays the bleeding body in the path, that the warriors may step over it, thereby believing that they will gain immunity for themselves in the approaching combat."
Great African Travellers from Mungo Park to Livingstone and Stanley W.H.G. Kingston