Move your mead to secondary when the gravity is about 1/3 of your starting gravity. This is not a perfect rule, as every recipe is different. You can also check the gravity over the course of several days. If you see little to no change over three days, fermentation is complete.
3:5011:00Primary vs. Secondary Fermentation: What's the Difference? - YouTubeYouTubeStart of suggested clipEnd of suggested clipAgain so secondary fermentation is simply when the fermentation is still going but the primaryMoreAgain so secondary fermentation is simply when the fermentation is still going but the primary stages ended so oftentimes between primary and secondary there's a racking.
Once in secondary, stir vigorously or use a wine whip, until all of the sulfur dioxide is released (rotten eggs) from the mead. Put the airlock in place and let the mead to clear for at least 2-3 months. This can sometimes take longer.