I have booked through both the website and with the United Connection software; both have their elements of tedium but I much prefer the United Connection overall (I think it gets you some mileage -- but less than using the website; maybe someone else can confirm).
1. On the website, simply build the itinerary point to point, six segments, paying attention to having at least 2 hours between flights (you can pick the flights you want after this with less connection time, but the system seems to need that two hours initially to even come up with choices). After you've done your six segments the system grinds for awhile and then spits out the itinerary with a price you wouldn't have expected given what the previous screen said were the "As low as" prices.
2. Using United Connection software, simply enter SMF-ONT and then keep pressing the "More Flights" button. After it exhausts all possible one connection routings (with every carrier), it will then show you three connection routings, first on UA.
(There is a site maintained by one of the FT gurus where you can download the United Connection software, but I don't recall the URL. A search on the United Board will come up with the link pretty quickly, I'm sure.)

Finally, IF you want to know more quickly what routings are permissible and then use a human being to ticket it, download (and keep it current each month) United's timetable from their website. It will let you quickly see non-stop, direct, 1 and 2 connection routings.

Hope that helps.