How should I use this skin-brightening cream containing alpha arbutin, kojic acid and tranexamic acid?

Generally, a cream like this is applied to clean, dry skin once or twice a day, in a thin layer over the affected areas - going heavier doesn't speed things up, it just raises the chance of irritation. Since it's meant for pigmentation, daytime sunscreen really is non-negotiable, or you'll undo the progress. Do check with your dermatologist about the exact frequency for your skin type before you start.