How do they, in Tantra Buddhism, invoke a deity and can someone explain the process of transformation?

What is the process of transformation and deity invocation in Tantric Buddhism? Can someone explain the process? Thanks!|||"The blows of life transform us. Life will give us many forms. After Heaven and Earth exist, individuals develop to fill a space in-between. The birth is always difficult. We all need help to change and grow, and what if there is no help? No failure is forever. There is always change, and a new beginning."