Others have some nice descriptions how to do this:

• Xilinx ISE WebPack for Linux: http://lug.wsu.edu/node/383
  • Comments here: http://lug.wsu.edu/node/204
• Xilinx Webpack v8.1i on Gentoo Linux with a 2.6 kernel: http://gentoo-wiki.com/HOWTO_Xilinx