Firstly, come up with a good idea and build it! Make sure to follow the Building Standards and make a building that isn't already on the game. It's most likely to be accepted if it is not something that somebody else already submitted.
Secondly, choose a good time to submit your idea. If we are working on prehistoric items and you submit a space station (Not that we would accept that) your model may be forgotten. Try to submit something approriate for the things that we are working on at that time. This is more likely to be added to the game.
Thirdly, suggest adding your building to the game! Do this by either posting it to the group wall or putting it on the ideas form (While following the guidelines of course). For the second option just put "I made this model" with a link on the bottom of your post. If absolutely necessary then PM me, Fredtron or the creator, Jobro13. Follow this page to get the best bet of getting your building on to the game.