I'm not aware of any particular instructions for this make / model, but, when I put OpenWRT on a router a couple of months back, it was a simple process of downloading a firmware image, and installing it via the "firmware update" function on the device. When it came back up, it was running OpenWRT, and, logging in through the web interface with the default username and password, it was pretty intuitive as to how to configure it.
My recollection is that all OpenWRT releases support USB modems over any available USB ports, even if the original software build did not support this.
OpenWRT is a fantastic system, especially if you are comfortable configuring devices via command line, but the GUI is really rather impressive. However, it's still - in my view, at least - not really for the fainthearted, and you would want a reasonable degree of confidence in being able to fix something if it were to go wrong before opting for it over default firmware.
In terms of a guide, if you cannot find something suitable on the OpenWRT site, a forum dedicated to OpenWRT might offer more responses than here — OpenWRT is not something which comes up on here very often.
Neil
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