This is a monster subject, and I spent a long time scratching my head over a few things yesterday, until I came across this site:
It’s developed by a Symfony user, and allows you to type in sql statements using nested braces for prioritising. The form will then return a nicely formatted block of Propel code.
It’s not perfect, and it doesn’t cover everything just yet, but it’s great for getting some ideas about the structure and how to use various criteria – it’s also spot on if you are trying to learn Propel and already know how to structure SQL statements.
Discuss this tool and/or contribute on the Symfony Forum
Using a combination of tools like this, and taking a look at the generated files in lib/model/om should give you a good push, and a bit of a shortcut to more complex queries than you may be used to – rather than having to trawl the Propel documentation.