Find a spot, design a castle, on the customize page of house menu at the bottom you will see a thing to click to put the castle into the contest. Make sure the house is public. Even if you customize the house after you enter it will be updated. I think somewhere around the 1st is when the voting starts. You may get the Heritage tokens and have add ons that way but they will not carry over to the design if you happen to win. (just say give tokens, I think there are about 10 you will get.)
Mine is located in fel where our Gilfane Museum is located.
Good luck.