First, we place the compass at A. Using a compass setting that is greater than half the length of we draw an arc like below.
Keeping that same compass setting, we draw one more arc having placed the compass at We draw the second arc so that it intersects the first one at two points.
Finally, we draw a line through the points of intersection. This line is the perpendicular bisector to It passes through the midpoint of so
Let's show the finished product.