Changing stealth settings from the front page of a topic
Turn editing on in your topic
For the activity/resource you want to stealth, in the pull-down menu for Actions select 'Hide'
The activity/resource will now say 'Hidden from students'
For the same activity/resource, return to the pull-down menu for Actions. This time select 'Make available'
The activity/resource will now say 'Available but not shown on topic page'. When a student visits the page, they will not see the activity/resource but if you provide a link to it, they will be able to access it
Changing stealth settings from an edit settings page
Alternatively, if you are editing the settings of an activity or resource:
Open the Common module settings section
Go to the Availability menu and select Make available but not shown on topic page.