(If you want to go directly to summoning an entity to aid you - skip this section and go directly to section 13 onwards).
Negative entities exist in dimensions called Talas in Hinduism. (Positive energy/beings exists in Lokas). The process of calling an entity is to create a resonant bond between your Atman (soul) and the being you want summoned. For example, to summon a Preta (Spirit) you must link your Atman resonantly with the Preta-Tala that the spirit exists in.
This 'linking' can be achieved by several tried and tested methods perfected by the Tantric Aaiyyanists of the Abhichaara school. These methods are said to date back thousands of years, and each methods is specific to the Entity/Tala in question.
To put it simply: the Dimension (Tala) you open has a limited amount of time/energy before it closes again. The period of opening is determined by how 'linked' you are to the entity you wish to summon. Some Tantric Aaiyyanists are very 'linked' to their entity as they have met them personally in previous incarnations. Other humans/beings can become resonantly linked to particular entities as they develop their relationship.