You can create a new pipeline from the left navigation of Gmail, under the Streak menu:

Clicking the plus button to the right of Pipelines will open a new window that allows you to select a pre-made template. 

We recommend picking one that fits how you're planning to use Streak, but don't stress if you've picked the wrong one: you can always create a new pipeline or customize the pipeline further.

After choosing, you'll be given the option to share the pipeline (a choice you can make later, too, if you're just getting started) and to finally create the pipeline.