From Ground: Dig the soil around the citron plant's base with a spade or trowel, keeping a good distance to avoid harming the root system. Carefully work the tool under the root ball to loosen it, take as much root system as possible while lifting the plant out.
From Pot: Gently tap the pot sides to loosen the soil, and invert it while holding your hand on the soil surface with the citron plant between your fingers. The plant should slide out with the soil and roots intact.
From Seedling Tray: Use a small spoon or similar tool to lift the seedling out, again taking care not to damage the young roots. Handle the citron plant by its leaves rather than its stem to prevent crushing.