Measure one side of the triangle and find the midpoint.
Line up the center of the protractor with the center of the side.
Mark the 90-degree point over the center point of the triangle side.
Draw a line connecting the center point of the triangle's side and the 90-degree point. This line is perpendicular bisector of the side of the triangle.
Repeat Steps 1 through 4 for one other side.
Extend the lines until they cross. If the triangle is obtuse, you will have to extend the lines outside the triangle.
Place the point of the compass on the point where the bisectors cross.
Pivot the pencil while holding the point steady in the center of the triangle. Draw a circle all the way around the triangle, touching each of the triangle's vertices.