And the video said that use shift+ctrl ,you will get a straight line
Just shift should do. Click the mouse to start sketching the spheres, slowly start moving them towards the direction you want, and then hold shift to snap it to a straight line. Smoothing afterwards can further make things taught.
You may also want to try using the Armature tool, or with Sketch1 activated go to Picker: (Depth) and set it to Once Z. By default Sketch1 tries to hug the surface, which is why you get those dips around the arm pit. With it set to Once Z or with the Armature brush selected, that behavior should be disabled.