How to ask for a recommendation letter from a professor after graduate a long time?

In US for graduate schools admission Universities require students to submit 2 to 3 letters of recommendation. It should be given by lecturers, managers or professors who had worked with  the students, observe the skills and provide recommendation based on their observation. These letters help the colleges to get perspective on your background and strengths. A student can get it even after a long time since he graduated, he/she should contact the professor for this.