To generate logs: "Completed operation A"
Logger.debug fn -> inspect(somevar) end
Logger.error "Some exception #{somexception} occured"

For projects generated with Mix, you can specify some options in config/config.exs:

use Mix.Config
config :logger, 
  backends: [:console] # default, support for additional log sinks
  compile_time_purge_level: :info # purges logs with lower level than this

A full range options are available in the official Logger documentation.

Additional reading: