Once I promised to allah to do a certain thing . But I couldn't keep my promise. What is the process of kaffara for this ?

Salaams,

Islam Question and Answer - He has the habit of swearing by Allaah whether he is telling the truth or not. How can he expiate for these oaths?
Islam Question and Answer - Ruling on offering expiation for breaking an oath (kafaarat yameen) in the form of money
Islam Question and Answer - There is no expiation for a false oath except sincere repentance

Hope this helps.

.

Salaam,

Thank u very much. May Allah bless you.