The 'AppData' installation directory didn't show up in PyCharm because it's hidden.
...so I navigated to the directory in Windows Explorer and copied the path.
I then copied that path into PyCharm and added '/python.exe' to the end, and it found the file.
Inherit global site-packages: Leave this unchecked.
Make available to all projects: I generally leave this unchecked.
Click 'OK' in the 'Create Virtual Environment' modal.
You should see a screen like the one in the image.
Click 'OK' to save the changes.
Set up PyCharm to start the virtualenv every time you start a Terminal session.
Background / Explanation
We need to do this so that we can generate the 'requirements.txt' file (a later step).
Step-by-step instructions:
On the left-side folder-view pane, navigate to 'your_project/venv/Scripts/activate.bat'.
Right-click the file, and select 'Copy Path'.
Go to File → Settings → Tools → Terminal → 'Shell path' and add "/K path/to/activate.bat" after the "cmd.exe". (Source)
Restart PyCharm.
Start a Terminal session and you should see the name of your virtualenv in parentheses on the command line. Running 'python' should start the version of python that you set earlier.
Create a 'requirements.txt' file in PyCharm.
Explanation:
We're doing this so that we can have our (not-yet-created) PythonAnywhere virtualenv download the necessary packages.
Step-by-step instructions: (You can find official instructions here.)
Start a Terminal session and run 'pip freeze > requirements.txt'. (Source)
(As you can see in the screenshot, it may prompt you to upgrade pip first.)
The file should appear, and when you open it it should have a list of packages and their versions.
Use SourceTree to push new files to the Bitbucket repository.
If you switch back to SourceTree you should see a list of files that SourceTree has noticed as being new.
Ignore the '.idea/' folder and the 'venv' folder.
We don't want to have our PyCharm files in the git repository (I think), so navigate to 'Actions' → 'Ignore'
Select 'Ignore everything beneath: .idea'
Once that's done those '.idea' files should disappear from the list of Unstaged files.
Explanation: The command to activate a virtualenv on PythonAnywhere has global scope (is that the way to describe it?), so you can't name all the virtualenvs "venv"; you'll have a name conflict.
So your virtualenv on your computer will be named "venv", and your virtualenv on PythonAnywhere will be named after your project.
Change the version number at the end ("python3.5") to the version of Python you'll be using.
Note: It looks like specifying the minor version doesn't work. I tried "3.5.2" and it produced an error, but "3.5" worked.
Install the package requirements in your PythonAnywhere virtualenv.
Activate the virtualenv with "workon name_of_your_virtualenv".