Again the direction of the extrusion should be away from the block. Now you can choose a surface or plane for the reference orientation. Pick DTM3 as the bottom. Turn the Grid Snap Off. In the LINE TYPE menu there are a list of several way to draw lines.
Center lines are just a different kind of referencing for dimensioning sketches.
Trace DTM2 and DTM3 with centerlines. Alignment allows you to align objects with other objects such as Datum planes. Pick the horizontal centerline you just drew and then pick DTM3. Repeat the previous step with the vertical centerline and DTM2. Go back to Centerline.