The Pro Git Installing Git article says:
If you can, it's generally useful to install Git from source...
Phil Haack's blog post on Posh-Git says:
Once you have Posh-Git loaded up, your PowerShell window lights up with extra...
And the Posh-Git repository installation instructions gives high-level install instructions.
Together, these three links give a high level map of setting up and using Git locally but do not go into details.
So, what we want is a a step-by-step guide to installing Git, installing Posh-Git, and then loading up Posh.