Download the program from the downloads page.
The setup program will expand all the files needed for WordSmith Tools into the folder of your choice (\wsmith7 by default). You can install to a removable drive if you wish (explained below).
Now run \wsmith7\wordsmith7.exe to get started. You will be asked to register. Otherwise WordSmith will go through its paces as a Demonstration Version.
Install on a USB stick?
WordSmith will run happily from a memory stick or external drive. Many users find it convenient to install on an external USB drive or memory stick. That way you can easily take it with you and run on a lot of different PCs. Many universities do let software run from a USB drive even though they prevent their users from installing. More details here.