I created a project using rebar called "mything". I added lager as a dependency. Now how do I configure lager? I tried adding a "lager" section to "env" in mything.app.src but it doesn't seem to get those settings. I guess I don't know where env settings for dependencies are supposed to go.
You need to create config_name.config file which contains list of configurations for each appliaction you want to configure:
[{mything, [...]},
{lager, [...]}].
Then you can pass it to erl using option -config config_name. It's called system configuration and it overrides default environment properties from .app files. In releases it's usually named sys.config.
Resources for you: Configuring an Application and config.