The Ripple Wiki has a section on running a Gateway.

Basic summary:

• Determine what you will be a gateway for (e.g. bitcoin, alt-coin, fiat, collector coins, etc)
• Accept those items outside of Ripple (via a website, in person, via mail, etc)
• Issue IOU in Ripple for items received (make a Ripple payment)
• Publish and follow a redemption agreement; what you promise to do when sent back your IOUs
• When sent a payment in your own IOU, you return/send the items outside of Ripple as per your redemption agreement (via a website, in person, etc)

There is a good example of a private party starting and running a gateway for trading DYM (pre-1964 silver US dimes).

Depending on what you are a gateway for, you can do the Ripple interaction manually (e.g. for the US dimes above it's all manual since the person needs to evaluate and handle incoming dimes anyway) or you can automate it (e.g. there is a TRC and LTC gateway that uses a web interface to configure a Ripple address to TRC and/or LTC address mapping and then automatically passes funds into and out of Ripple.