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? (http://www.islam-qa.com/en/ref/36734/expiation)
Islam Question and Answer - Ruling on offering expiation for breaking an oath (kafaarat yameen) in the form of money (http://www.islam-qa.com/en/ref/124274/expiation)
Islam Question and Answer - There is no expiation for a false oath except sincere repentance (http://www.islam-qa.com/en/ref/100449/expiation)

Hope this helps.

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Salaam,

Thank u very much. May Allah bless you.