Then select the "Manage Flags" tab on the opened modal.
This will give you access to the flag manager. First select the type of flag you want to apply:
You will see the full list of your project grids, sorted by models and sheets.
Click on the arrow next to a model or sheet name to expand/collapse it.
Then simply check to match checkbox to apply the selected flag to the selected grid(s).
You can add dynamically some flags like Resize flags, Groups flags, ... Simply click on the Add flag blue button.
Tips
To navigate through the flag manager, you can use the search field in the top-left corner.
You can also download a summary of all your flags by clicking on the download button.
One last tip, if you want to display only the grids that are flagged with a specific flag, simply click on the flag name.