Edges and +centres: Start at UB - that's A. Then go clockwise around the face, so UR UF UL are B C D. Then go to the L face and make the same pattern. Then do the same pattern on the F, R, B and D faces in that order.
Corners and xcentres: ULB is A, then it's the same deal - clockwise around each face in the order ULFRBD.