HERMES is a brand encompassing three products under one metaphorical umbrella, each being written by different teams in different languages. Accordingly, the contribution workflow is broadly similar across products (for example, Mercurial is used to track and save changes), but the process of compilation differs wildly. Relevant documentation is included in each repository.
To get started, download Mercurial if you don't have it already, and issue the shell command appropriate to the product you wish to develop:
hg clone https://bitbucket.org/team-hermes/hermes-mail
Alternatively, if your preferences run towards graphical tools, you may download Sourcetree instead