Go to https://xyz.atlassian.net/wiki/admin/viewgeneralconfig.action (replacing xyz with your instance sub-domain) and click on "Gliffy - draw.io conversion" in the menu.

Click "start import" and wait for the import to complete. The total time required depends on the number of Gliffy diagrams in your Confluence. Keep your browser tab open while the import is in progress and ensure you device won't go to sleep/run out of battery shortly after starting the import.