This will not delete the current Sony Ericsson Home UI, it basically works "instead" of it if you install it (and when prompted if you set it as your default) -- you can safely uninstall it if you do not like it.
- Make sure you can install 3rd party applications on your device
- Do you have a file manager already installed? I recommend ES File Explorer, available from the Market
- Download HelixLauncher Donut, browse to the download location and run the APK file using a file manager