well first you need to have the following
sandpaper
laquer or clearcoat(paint but i dont recommend)
some kind of thick sting to hang the board on/long peice of lanyard(pivit cup)
So first sand it till it perfectly smooth on the bottom,sides and top if you want to refinish the top
Once you do that, hang the board in the air or if you dont wanna then put it down on something were the bottom is up/top is up(depending on witch side is being redone)
The evenly spread a thin coat of the laquer or clearcoat on and repeat 4 or 5 times
If you cant get clearcoat/laquer, then you can paint it but its not the best idea to do it unless you have primer and the normal paint
so thats it