How can I use templates for projects and activities?

General Questions
You can add as many templates as you like then create projects based on templates.
To make a project a template just assign it the "Template" status.
Templates are not shown in the Activity Center (unless you enable the filter to show projects with the Template status) and in reports.
 
To create a project based on a template go to the Activity Center, click on the "Type here to create a new project or activity" field, type a name of your project, select "Project" in the "Create new" field and choose a template from the "based on template" drop-down list.
 
 
Note that you can use any project or activity as a "template" and duplicate it when needed.
It can be done in the Activity Center using the Duplicate button in the batch operations control panel.