The proper answer to this question is that pipenv install or pipenv install --dev (if there are dev dependencies) should be ran. That will install all the dependencies in the Pipfile. Putting the dependencies into a requirements.txt and then using pip will work but is not really necessary. The whole point of using pipenv for most people is to avoid the need to manage a requirements.txt or to use pip.
EDIT: if the virtualenv is already activated, you can also use pipenv sync or pipenv sync --dev for the same effect.