1) Drink, drink a lot (of water)
Alcohol is a powerful diuretic, you will have noticed it. In addition, if you have rejected your deepest thoughts from your stomach, you have also lost a lot of water and minerals. Drinking a lot of water the next day is very important, especially if slightly sparkling and in any case, room temperature, better still if sweetened or enriched with mineral salts: the ideal would be the typical healthy drinks of sportsmen, the energetic ones. Alternatively you can try lemonade, or an infusion of warm water and lemon peel (the somewhere called canary): this will also help you throw out any residuals, if you still have any. It seems that in addition, drinking a nice cup of milk is just as useful to protect the walls of the stomach and give you some relief. Probably your stomach will rebel, but rest assured, today you will rebel against anything.

2) Help the body to absorb alcohol 
I imagine (from personal experience) that you are not hungry. Indeed, the idea of ​​eating makes you feel even worse. But eating in this case is very useful, because the intake of food, especially starches and carbohydrates (potatoes and pasta), and bananas, which compensate for the loss of potassium, help the body to absorb and split the molecules of alcohol. And many (perhaps the bravest) swear to have had relief from hangover by eating a full healthy meal, whether this is a nice stuffed sandwich or a hot hen broth.

3)Have some sleep
Whatever you planned to do, forget it. If possible take a day off or sick, because sleeping in this case will only do you good. You will also sleep for 12/13 hours, but surely you will feel a little better when waking up (last time I slept for 10 hours and I thought they were 24).

4)Try to sweat
Eliminating toxins and evaporating alcohol sweating is a good idea, just maybe you do not want to go out jogging in the park leaning every 4 meters to a different tree. Or maybe you have already followed point 3 and you are in the dream world. Then maybe a very hot bath will help you, even before getting back to bed. Just try to avoid joining point 3 with the hot bath.

5)Take an aspirin or painkiller if your head is about to explode
Warning: personally I do not recommend taking medicines, simply because with the stomach and the body already in rebellion, perhaps taking aspirin is not exactly the best. But if you really have to go to work, or if you can not stand the headache that is tormenting you, take an aspirin or a painkiller and let it calm down your symptoms.

I wish you good luck and reassure by telling you that you are not the only one: maybe at this very same moment millions of people are in your same situation, slaves of the annoyances of the day after New Years Celebrations.