To start a new checklist go to the template page for which you want to start a checklist and click on Start checklist from the action menu.
This will open a flyout where you can change the name of the new checklist and select which persons from the team is going to be assigned to the new checklist.
Start the checklist by clicking on the Start checklist button.
The new checklist is now created, the people are assigned and you can start completing tasks.
Alternative way to start checklists
Another way to start a checklist is to go to the team page.
Click on the action bar on the template you want to start a checklist from.
Click on the Start checklist button on the left side in the action bar. This open the flyout where you name your new checklist and assign people. Click on the Start checklist button to start a new checklist.