Turbulator tape improves low speed handling characteristics by delaying flow separation from the wing. This is achieved by the tape producing tiny vortices that disrupt the separation bubble. The vortices have the effect of suppressing the separation bubble and allowing well attached air flow ahead of and behind the turbulator; resulting in lower stall speeds, lower drag and reduced tip-stall tendency. Turbulators have also been used in front of control surfaces to improve low speed effectiveness.
While at high speeds turbulators do add a small amount drag, they actually reduce drag at lower speeds due to improved air flows.
The position of the tapes on the wing is found by trial and error; unless you have access to a wind tunnel. A general rule of thumb is to start at 20% cord on the top of the wing and 40% on the bottom ( turbulators on the bottom of the wing are less common).