For starters, make sure the downloaded lots are compatible with your game expansion(s), if any. For example, don't download a lot that requires, say, Open For Business if you don't have said expansion. Second, get Clean Installer (do a google search and you'll find it, can't remember the link right now) so you're sure you're installing the house ONLY and not all the rabble that could come with it.
Can you be a little less specific? That's too much detail.
I second that!