Beat the egg whites
Fit a stand mixer with the whip attachment or a handheld mixer with the twin beaters. Beat room-temperature egg whites with a pinch of cream of tartar (for stability) on medium speed until foamy and the cream of tartar dissolves, about 1 minute.
Look for soft peaks
If your recipe calls for egg whites whipped to the soft-peak stage, beat on medium-high speed until the whites look opaque but moist, 2 to 3 minutes. Stop beating and lift the whip: The whites should form slightly bent peaks.
Beat to stiff peaks
If your recipe calls for egg whites with stiff peaks, continue to beat until the whites look glossy, 1 to 2 minutes longer. When the whip is lifted, they should hold a firm, straight peak. Take care not to overbeat.
Overbeaten egg whites appear grainy and can separate. If you beat the egg whites to this stage, you’ll need to discard them and start again with fresh egg whites.