Plop the sporran about 1-2 inches below you belt and hold it there. The chain should be attached on one side of the sporran. Take the other side and swing it around to the other clasping ring. The excess in the chain is where you'll need to chop it off.

If you don't have the tools, mark where you need it cut and go to Home Depot or Lowe's and beg them to give it a snip. They'll be more than happy to do it.