In addition to the dependencies specified in Installation, noting that the python dependencies can be installed with pip, which comes with Python, you will need to install the following dependencies:
https://www.python.org/downloads/windows/
Python provides an option to add it to your PATH environment variable during installation, which you should select (otherwise, you must add the python scripts directory to your PATH).
Git
https://git-scm.com/download/win
While a dependency for all systems, it generally is not already installed on Windows.
By default, Git will add the necessary directory to your PATH (the "recommended" option), and as long as you do not choose the "Use Git from Git Bash Only" option when installing, Portmod will be able to find your Git installation automatically.
7-zip
7-zip is optional, and can be replaced with other archive tools which are supported by patool (unfortunately, no nicely formatted list exists, but see the repo for supported programs).
7-zip is recommended as it covers most archive types not supported natively by patool, including the commonly encountered types (see note on patool in dependencies section on the Installation page).
The 7-zip program files directory needs to be added to your PATH environment variable so that it can be detected by patool.
Sandboxie
Finally, you will also need to install Sandboxie, the Sandbox tool Portmod interfaces with on Windows (see Sandbox). Note that Portmod automatically configures Sandboxie; you do not need to, and indeed should not, modify the sandbox configurations generated by Portmod.
The sandboxie program files directory needs to be added to your PATH environment variable so that it can be detected by patool.
Setup information
It is worth noting that, while Portmod in general does not require administrator privileges, you will need administrator privileges to create symlinks on certain versions of Windows (newer versions of Windows 10 should support creating symlinks without privileges). Currently the only symlink portmod requires is the profile symlink, so when you select a profile using omwselect profile set, you will need to ensure that omwselect is run with administrator privileges.