Not too difficult a task. Call and ask for an appointment (or invite for coffee). Explain your aspirations, what you've been doing since graduation, and then ask for a recommendation. Most importantly, offer to give them a first draft of the letter of recommendation (this is very common, especially within academia; it also allows you to demonstrate your strengths in a fashion you think AdComs will value). Be sure to offer your complete transcript, an updated resume, and possibly your personal statement. You might also do well to share with them any written products from the classes you took.
If they accept, provide everything in hard copy, give them the draft in electronic copy, and give them a stamped and addressed envelope with the LOR form filled out and ready.
And try to be low-impact. They get hit up for LORs right and left...make it as easy as possible.