View the documentation and tutorials at flexget.readthedocs.org
Making custom plugins should be easy for anyone with some python experience.
If you're working on good re-usable plugin we're be more than happy to include it in official distribution. See Contribute for more information.
FlexGet has over 1200 unit tests so chances are that if your modifications pass the tests nothing major has been broken.
First install IPython
bin/pip install ipython
And then place this where you wish to hack:
import IPython; IPython.embed()
Thanks to JetBrains for the free open source PyCharm license!