Zest citrus fruit *before* juicing it. It's much easier to zest a firm, whole fruit than a flattened, slippery half.
Zesting citrus fruit before juicing offers several advantages. First, it provides a stable surface to work with. A whole fruit is much easier to grip and control, allowing you to apply even pressure and avoid accidentally grating your fingers. Second, the zest is easier to remove from a firm fruit. Once juiced, the fruit becomes soft and pliable, making it difficult to apply the necessary pressure for zesting without tearing the peel. Finally, zesting first prevents the juice from spraying everywhere while you're trying to zest.
When zesting, avoid grating the white pith underneath the colored peel. The pith is bitter and will negatively impact the flavor of your dish. Only zest the outermost layer of the skin.