In the figure attached, red circle A and red point B are the circle and external point of interest. Note that we must know where the center of circle A is. If we don't know that, there are construction techniques for finding it, but that is beyond the scope of this answer.
Step 1. Set your compass to a radius greater than half the length of segment AB. Here, we have made the radius AD.
Step 2. Draw arcs above and below the center of segment AB centered at A and B using the radius of Step 1. Here the "arc" is shown a a full (green) circle. Only the points where the arcs intersect (E and F) are of interest, so it is not necessary to draw the full circle.
Step 3. Identify the points of interesection (E and F) of the arcs of Step 2, then draw a line segment between them. This segment (EF) is the perpendicular bisector of AB. Mark point G where it intersects segment AB. As with the green circles, it is not necessary to draw the whole line EF, since we are only interested in the location of the midpoint of AB, which is point G.
Step 4. Using G as the center, and GA or GB as the radius, draw semicircle AHB. The point of intersection H is the only part of that (blue) circle of interest, so it is not necessary to draw the whole thing.
Step 5. Finish the consruction by drawing tangent line BH.
Step 1. Set your compass to a radius greater than half the length of segment AB. Here, we have made the radius AD.
Step 2. Draw arcs above and below the center of segment AB centered at A and B using the radius of Step 1. Here the "arc" is shown a a full (green) circle. Only the points where the arcs intersect (E and F) are of interest, so it is not necessary to draw the full circle.
Step 3. Identify the points of interesection (E and F) of the arcs of Step 2, then draw a line segment between them. This segment (EF) is the perpendicular bisector of AB. Mark point G where it intersects segment AB. As with the green circles, it is not necessary to draw the whole line EF, since we are only interested in the location of the midpoint of AB, which is point G.
Step 4. Using G as the center, and GA or GB as the radius, draw semicircle AHB. The point of intersection H is the only part of that (blue) circle of interest, so it is not necessary to draw the whole thing.
Step 5. Finish the consruction by drawing tangent line BH.