Dashboard templates
Under Select Dashboard choose Edit for the dashboard you want to use as a template dashboard.
In the details dialog, make sure to check the Combine template variables checkbox and save.
The template dashboard will now show a combined form where you can set all template variables and choose assets for the asset place-holder template variables.
Filtered results The asset selection dialog has an intrinsic filter query such that you can only pick suitable assets.
Template variables that have the same name and type will be combined. They only show up once in the dashboard template form and will be used in all report templates that share the variable. All the connected report widges will be filled with the chosen variable.
The dashboard features a header in the top showing the selected
template variable values. 
The header also allows you to change those values at any time. Simply click on the variable value you want to change in order to edit it. For asset place-holders, this will bring up the selection menu again.