Summary

Download the sim file.
File in zip or rar format, containing a .Sim file?
Extract from the zip or rar.
Place the .sim file in the SavedSims folder in:

If you do not have a SavedSims folder, make one in the above directory.
File in sims3pack format?
Go to Installing Sims3Pack
Detailed Instructions

Don't understand based on the summary? Here's the "for dummies" steps:


Step 1 - Get a ZIP/RAR Extractor
First, you will need a program that can extract from both ZIP and RAR files.
Windows
We recommend WinRAR, which is "nagware". That means that you can use it for free for a period of 45 days, and after that you can continue using it free - it just nags you to pay for it, but you don't have to pay anything to use it. Download the installer and run it.
OS X
Zip/unzip functionality is already built into OS X (right-click and select "Open"). The free utilities StuffIt Expander and The Unarchiver handle most any archive type. For .rar files in particular, you can also use UnRarX, a simple and efficient freeware tool.
Then, once your preferred de-archiving application is installed...

Step 2 - Download the Sim File
Click on the sim file link. Save the file to somewhere you can locate it easily - your computer's Desktop is the easiest.

Step 3 - Extract the Sim File
Once the file has finished downloading...
Go to where you saved it (like your computer's Desktop).
Windows
If you got WinRAR, you should be able to just right-click on the file and choose Extract Here. Then you should have a new file below the RAR.
OS X
On OS X, double-clicking on an archive will extract its contents in the same location.

Step 4 - Identify File Type
Now, you need to know what type of file you have. The easiest way to do this in Windows is to right-click on the file and choose Properties. On the window that pops up, it will say "Type of file:" and then some text after that. For sims, there are two options on file types they might be: SIM files or SIMS3PACK files.

Step 5 - Install the Sim File
If you have a SIM file
If the file you got in step 4 is a SIM file type...
Right-click on the SIM file you extracted in step 3 and choose Cut.
Then click on Start - My Computer.
Navigate to SavedSims in

Right-click in an empty space in the SavedSims folder and choose Paste. The SIM file should appear there.
If you do not have a SavedSims folder, make one in the above directory.