1. Go to Edit Town
2. Click on the house
4. Click on Edit Details (far right) and give the house a name and a brief description. The game will use the name of the house as the file name.
5. Select Share (this will put a copy in your Export folder as well as a copy in your library)
6. You can now attach the file using the "Attachments and other options" in your post on these forums.
7. The file will be found in your My Documents > Electronic Arts > The Sims 3 > Exports folder and it will be in the Sims3Pack file format.
That's the quick and dirty on houses.