If you're using the public Galaxy web-site, FASTX-toolkit tools are already available for general use.
If you're running a local mirror of the galaxy server, follow the Installation instructions to integrate the tools into your Galaxy server.
Inside Galaxy, click on FASTA/Q Information or FASTA/Q Manipulation categories to access the FASTX-Toolkit tools:
Each tool contains a optional parameters and a short description. Click on the different tools to see their help screen.