Considering that |\frac{a}{b}|= \frac{|a|}{|b|}, you get that |\frac{m}{8}|=1 if and only if \frac{|m|}{|8|}=1.
Now, obviously |8|=8, since 8 is a positive number and the absolute value of a number is the number itself is the number is positive, and the opposite otherwise.
So, we can rewrite the equality as \frac{|m|}{8}=1, which yields |m|=8
This request has two solutions. In fact, if m is positive, then |m|=m, and so we have m=8.
Otherwise, if m is negative, we have that |m|=-m, and thus m=-8 solves the equation.
We conclude that |\frac{m}{8}|=1 if and only if m=\pm 8