Step 1. Make a collection.
Step 2. Exit the game. (Do not make multiple collections to share at once- their files are not named and you may get confused when sharing them.)
Step 3. Go to MyDocuments/ElectronicArts/TheSims3/Collections/User
Step 4. Arrange the folder by Date Modified. Find the package file that matches the time you last added something to your collection. It should be the most recent file.
Step 5. Change the name! You can rename collection files. Give it a really good descriptive name so people know what they've got.
Step 6. Share it!
All it takes to share the collection is that one single file. Another player sticks it into their Collections/User folder and it appears in game! You can use objects from any expansion/stuff pack and even custom content- the objects simply won't appear in the collection if the other player does not have them.