Imagine someone perform an action. What are the requirements should be completed for it to be a complete sin?
Explain it by using killing an animal as an exaple
I don't think it's thoroughly explained in suttas.
I know you tagged this topic lay-buddhism, but it might be relevant to mention that the topic is extensively discussed in the Vinaya (the "discipline", the rules for monks).
Pages of 74 through 86 of The Buddhist Monastic Code says:
This rule against intentionally causing the death of a human being is best understood in terms of five factors, all of which must be present for there to be the full offense.
- Object: a human being, which according to the Vibhaṅga includes human fetuses as well, counting from the time consciousness first arises in the womb immediately after conception up to the time of death.
- Intention: knowingly, consciously, deliberately, and purposefully wanting to cause that person’s death. “Knowingly” also includes the factor of—
- Perception: perceiving the person as a living being.
- Effort: whatever one does with the purpose of causing that person to die.
- Result: The life-faculty of the person is cut as the result of one’s act.
It details a number of exceptions: if you kill a person by accident; if you thought they were already dead; or etc.
It also details some non-exceptions, e.g. telling someone else to kill does count as "effort".
Perhaps I could mention that attempted murder (where the victim doesn't die as a "result") is considered still an offence, and punished ... but with "lesser penalties".
The penalties (and the deeds for which monks are punished) in the Vinaya aren't necessarily the same as the working of kamma -- they're more well-defined (and so easier to list in detail).