Draw a triangle (using a ruler!) on a piece of cardboard. Cut it out. Find the centroid (intersection of medians). Balance the triangle on the point of a pencil.
On the same triangle’s reverse side, find the circumcentre (the intersection of the perpendicular bisectors). Remember that the circumcentre may not be on the triangle itself; it could be outside the triangle. In that case, just draw the perpendicular bisectors.
I made these diagrams using GeoGebra. It’s great.