For electron cyclotron current drive (ECCD) on burning plasmas in DEMOs a short millimeter wave is launched toward the plasma in the form of oblique O mode. The wave propagates to a target location and deposits its power into suprathermal electrons via a fundamental electron cyclotron (EC) resonance, generating a current. When the profiles of the target plasma are given, the problem is to find appropriate wave frequency , launcher location and launching angles to realize efficient ECCDs at target radii. We address this issue for a JA DEMO plasma as a test case. The angular toroidal wave number keeps constant during wave propagation and the parallel wave number is given by in good approximation. For each pair of and , we calculate the local wave damping rate and current drive efficiency at the fundamental and second EC resonances separately and make mappings of these quantities on the poloidal cross section. Information in these mappings is integrated into one chart. Any wave trajectory can be sorted by a pair of and and a series of integrated charts over and provides an overview of ECCDs in the target plasma. Guided by this overview, we search for appropriate pairs of and , and wave propagation paths to realize efficient ECCDs with negligible power consumption by second harmonic resonance on the way to the target locations at various radii. Thus, we find a number of pairs of and with appropriately adjusted propagation paths to design the wave launching system.