The arrangement of initials in a monogram depends on the style you choose. For styles where the center letter is larger or more prominent, the traditional order is first/last/middle. For example, if your names are Mary Ann Smith, the monogram would be MSA.
In styles where all letters are the same size, the order should be first/middle/last. In this case, the monogram would be MAS. Be sure to select the style that best matches your preference for how the initials are displayed.