Creating a custom template
Create the template content in a new board
Create a new board with the starter content you'd like for the empty version of your template. Note that it's not important where this template is saved (it can be anywhere in your workspace).
Right click to convert the board to a template
This is done from "outside" of the board you want to convert. Once you've done that, you'll see a new "Template" badge below the board title.
Using a template you've created
Drag out a new empty board from the toolbar on the left.
Double-click to open it.
Choose More templates... from the popup in the bottom right of the screen.
Choose Your custom templates.
Select the template you created earlier.
Choose Use this template and you'll get a fresh copy of it you can start editing.
Sharing templates with your team
You can share template boards with your team the same way you share any other board.
Any template that's inside a regular shared board will also appear in the templates popup for anyone who has access to it.