There really aren't any steps. It's just a handy fact about Isosceles triangles.
An Isosceles triangle is a triangle that has two sides that are the same length, and one of the facts about this kind of triangle is that the angles opposite the two equal sides are equal angles.
In your picture, the two sides of the triangle that have tick marks across them are equal lengths, and that means that the angles at 'S' and 'U' are equal angles.
In this kind of triangle, those are called the "base angles", even though they don't necessarily have to be the angles that the triangle stands on. They're just the two angles that are opposite the two equal sides.
