To construct a common internal tangent you need to remember what a common tangent is.
The line that is tangent to two coplanar circles is called common tangent.
There are two types of common tangent lines to two circles: common internal tangent and common external tangent.
How differs these two types of common tangents?
Draw a line joining the centers of the two circles then draw a common tangent.
If the common tangent intersects the line joining the centers of the circles, then this tangent is called common internal tangent.