The perfect tense is used to show that an action has (had, or will have) finished before a definite time or a second action. I'll give you a link to a diagram, where you can study each tense in present, past and future, and when to use it, just scroll down 'till you reach the perfect tense group: http://www.help2say.com/questions/990/how-to-learn-english-grammar-easily

I want to know how to use it in a sentence grammatically, would you explain it to me, please?

Do you want to know how to use it in a sentence grammatically, when to use it, or both?