It was usually thought that the person who had made a diabolical pact also promised the demon to kill children or consecrate them to the Devil at the moment of birth (many midwives were accused of this, due to the number of children that died at birth in the Middle Age and Renaissance), take part on Sabbaths, have sexual relations with demons, and sometimes engender children from an incubus or succubus (in the case of men).
The Malleus Maleficarum has plenty of allusions to these pacts, especially concerning women. It was considered that all witches and warlocks had made a pact with some demon, especially with Satan at least.
The diabolical pact can be of two natures, verbal or written.
A verbal pact is made by means of invocations, conjures and/or rituals to attract the demon; once the conjurer thinks the demon is present asks for the wanted favour and offers his/her soul in exchange, and no evidence is left of the pact; but according to some witch trials and inquisitions that were performed, even the verbal pact left evidence, the diabolical mark (an indelible mark), that could be used as a proof to determine that the pact was made.
A written pact consists of the same forms of attracting the demon, but includes a written act, usually signed with the conjurer's blood (although sometimes was also alleged that the whole act had to be written with blood, meanwhile some demonologists defended the idea of using red ink instead of blood and others suggested the use of animal blood instead of human).
These acts were presented often as a proof of diabolical pacts, but there is no proof of whether they were written by insane persons believing they were actually dealing with a demon or just were fake acts presented by the tribunals of the Inquisition. Usually the acts included strange characters that were said to be the signature of a demon, and each one had his own signature or seal. Books like The Lesser Key of Solomon (aka Clavicula Salomonis, Lemegeton) give a detailed list of these signs, known as seal of the demons.
A notorious case of fake acts was the one that cost the life of Urbain Grandier. One of the acts was redacted in Latin; the other is basically illegible, and signed by several "demons", one of them Satan, whose name was clearly written "Satanas".
According to demonology, there is a specific month, day of the week, and hour to call each demon, so the invocation for a pact has to be done at that time. As demonology says that each demon has a specific function, that one has to be invoked depending on what the conjurer is going to ask.