How do you use plantain as a medicine?

Chew it up. Common plantain, one of the most common lawn weeds, was known to native americans as snake bite weed. I don't know of anyone who has used it for snake bites, but it's great for insect stings. Take a leaf, chew it up until it's a pasty goo, then apply it to the insect bite for a few minutes. The sting/itch/swelling will fade away. Be careful you have identified it correctly first.