Arrange the numbers vertically and horizontally to create 5×5 grid and arranged the alphabet inside the grid but since 26 ang alphabet you should combine it sa iisang square ang I and J.

    1 2 3 4 5
1 A B C D E
  2 F G H I/J K
3 L M N O P
4 Q R S T U
5 V W X Y Z

Ex. 43-15-45-31
        S  E   U   L

Try to decode this one:
43-23-15-42-31-34-13-25...23-34-31-32-15-43

Credit to: ShinichiLaaabs