1.pick a color that's not white (I usually just use the same color as my shader
2. color where you want the highlights to be (on a layer above your flats)
3. a) using photoshop or gimp or something similar gaussian blur that layer so the lines you drew aren't sharp, but they don't blur into a large area (on a small canvas I usually do it at 34, on larger canvases 54 usually works)
3. b) using sai or something similar get a brush that fuzzes out the color for you. Like a water brush.
4. set the layer mode to divide or luminosity or something else that makes the color lighter.
5. on a new layer, retrace the places where you want sharper highlights to be seen.
6. put that layer mode to divide or luminosity or something, and you're finished!
What I did was (in GIMP though it should work in any program that has layers like PS or Sai) make a new layer and with a purple drew in where I wanted the shadows to go, then I set the layer to multiply or darken (I forget which, try them all~) then lower the opacity to around 60%. Then duplicate the layer and guassian blur it a bit (to get the smoother shading) and lighten the opacity of that to around 20-40%. Doing the highlights is the same except using soft light or lighten layer mode instead! c: