Question about many to many mapping in Entity Framework code first

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CodeFirst

entity-framework

Hello.
I have some trouble with add additional field in table, which Entity Framework created for me to solve many to many relationship.
Let's start from beggining.
I have 2 classes (models):

public class Craft
    {
        public int  CraftID { get; set; }
        public string Name { get; set; }
    }
}

and

public class User
{
    public int UserID { get; set; }
    public string UserName { get; set; }
    public string Email { get; set; }
    public string Password { get; set; }
}

I need to bind this two models with the following requirements:
1. Any user has all type of crafts.
2. For every craft user has it's own skills (example: for craft "Alchemy" user#1 has 12K Skills).
I've tried to resolve this problem using articles: Creating a Many To Many Mapping Using Code First[]. Entity framework created for me table "UserCraft" with "CraftID","UserID". But i need to add additional field "Skills" into "UserCraft". If I using DB first, i can create this table (UserCraft) by hand, create constraints, and add any additional field as much as I want. How I can do it with CodeFist?