I feel like some of the explanations on berkshelf on the internet are confusing.
So i felt like doing a small write up myself

berkshelf is pretty much a replacement for the “knife cookbook” command.
The big win with berkshelf is that it also resolves dependencies of a cookbook like apt or yum.
It reads a file called “Berksfile” for other cookbooks the current cookbook needs and what repositories to fetch them from and pulls them to your local system.

I will use the logstash cookbook at https://github.com/lusis/chef-logstash as an example
If you read the Berksfile at https://github.com/lusis/chef-logstash/blob/master/Berksfile
it will show you what other cookbooks the logstash cookbook needs

So in order to get going

gem install berkshelf
git clone git@github.com:lusis/chef-logstash.git
cd logstash
berks install
berks upload

That installed berkshelf, cloned the logstash cookbook, resolved dependencies for the logstash cookbook and uploaded logstash cookbook and its dependencies to your chef-server

Additionally berkshelf installs its configuration file at : ~/.berkshelf/config.json
You may need to edit some stuff there to match your ~/.chef/knife.rb file