How and on what speed do you beat egg whites and sugar to make them fluffy?

I agree with clean bowl room tempature egg whites on high to incorporate as much air as you can even a little drop of egg yolk would ruin them add the sugar slowly and you could also add a little bit of cream of tartar to stabilize them

I use a copper bowl and balloon whisk. Eggs done usually in 1 minute. Add the sugar just before they get stiff. The copper reacts with the egg to stabilize them.

Stick the bowl in the freezer before you begin, that will help.

Start with a clean bowl and room temperature egg whites. Beat the whites on hi - on my countertop mixer I use one click shy of full blast. Wait until soft peaks form, about 2-3 minutes then add the sugar slowly while beating. You should have firm peaks in 3 more min or so. Good luck!