To use Checklist for Salesforce, and after the button was added to the Page Layout (see setup),
just click it:
Add checklists for a record
This opens the Add Checklist popup - the current Tasks and their status are displayed:
View current Tasks and add select Checklist to add.
After selecting a checklist, select the items you want to add as Tasks:
Select, Review and Update Task information before saving.
The Open Activities Related List shows the newly created Tasks.
In addition to adding Tasks manually, you can also add Tasks automatically based on criteria.