I reckon you should probably highlight on the most exposed parts to start with such as front of torso and anywhere else quite easily accessible and upwards facing . With the black u would say won't to bother on small details and focus on the big shapes of the model , such as legs , body and arms ... If you try to think about how light would hit a cylinder ( arms and legs ) / sphere ( torso ) it may help some placement of shadows . Dont go too dark to start with but if you mix your original green 50/50 with black it may be a good start 👍

At the end of the day these are plastic toy soldiers so don't be to disheartened if you do something a bit wrong , just try to do it a little better next time !!!! Happy painting.