Your mother would need to apply at the U.S. consular post serving the area where she lives for a B-1/B-2 non immigrant visa, completing the DS-160 form online and paying the required fee. She would need to supply evidence of the purpose of the planned visit (something from the school officially stating your graduation date), your birth certificate to prove she is your mother, evidence that she will have a place to stay/funds for room and board during her visit, and evidence of her intent to return home at the end of her visit (job to return to, home to return to, other family or connections there).
Consult an attorney for further guidance.
Your mother will need to apply for a visitor visa, using Form DS-160. You can usually get instructions on the website for the US embassy or consulate, with links at email@example.com. Basically she'll need proof of a legitimate temporary purpose, such as an invitation or announcement of the graduation ceremony, proof of your legal status in the US, and compelling ties to Nepal, such as family, business and property obligations that would compel her to return upon the conclusion of her temporary visit.
Scott D. Pollock