There are three main components in a standard staircase.
Stair tread return length.
Double return stair treads have a mitered return on both the left and right ends.
Returns can also be added to existing treads as molding.
The stringer is the backbone of the structure.
Double return stair treads are always measured by the throat dimension the length between the two mitered returns.
Stringers are the diagonally placed boards anchored at the top the header and the bottom the footer of the staircase.
Because the mitered returns are both 1 wide the overall width of a double return stair tread will always be 2.
Stringers risers and treads.
Simply choose a length of return that matches the thickness and type of wood as the tread rout shape as desired cut to length and attach to the tread using a biscuit and glue join or a finish nailer.