You cannot compare fractions by looking at them — 5/8 and 2/3 have nothing in common until you rewrite them. Give them the same denominator, and then only the numerators matter. Watch the trap: a bigger denominator does not mean a bigger fraction. It means the whole was cut into more pieces, so each piece is smaller.
Worked example
Problem Arrange 2/3, 5/8 and 3/4 in order, beginning with the smallest.
Find a denominator they all share. The lowest common multiple of 3, 8 and 4 is 24.
Rewrite each one: 2/3 = 16/24, 5/8 = 15/24, 3/4 = 18/24.
Now only the numerators matter: 15 < 16 < 18, so the order is 5/8, 2/3, 3/4.
Watch out 8 is the biggest denominator here, but 5/8 is the smallest fraction. Never judge by the denominator alone.