We start with a triangle ABC.


1. Find the bisector
of one of the triangle sides. Any one will do.
See Constructing the Perpendicular Bisector of a Line Segment for detailed instructions.


2. Repeat for the another side. Any one will do. 

3. Mark the point where these two perpendiculars intersect as point O.


(Optional step) Repeat for the third side.
This will convince you that the three bisectors do, in fact, intersect at a single point.
But two are enough to find that point. 
Done The point O is the circumcenter of the triangle ABC.
Note: This point may be outside the triangle. This is normal.

