The same rules (pages 13 and 15) used in the major for finding the key having the next number of sharps and the key having the next number of flats are applicable in the minor. The order of the letters in both the sharp and flat signatures is the same in the minor as in the major.
A minor has no sharps, the fifth of a is e and has one sharp (f):—
e f# g a b c d# e
1 2∼3 4 5∼6∪7∼8
The fifth of e is b and has two sharps (f and c):—
b c# d e f# g a# b
1 2∼3 4 5∼6∪7∼8
The fifth of b is f# and has three sharps (f, c and g):—
f# g# a b c# d e# f#
1 2∼3 4 5∼6∪7∼8
The fifth of f# is c# and has four sharps (f, c, g and d):—
c# d# e f# g# a b# c#
1 2∼3 4 5∼6∪7∼8