When pairing a shirt with a cardigan, it's important to create a harmonious balance between the two pieces. Opt for a button-down collar or a classic collar shirt that complements the V-neck of the cardigan. Colors can be coordinated for a cohesive look, or you can play with contrasts by choosing complementary colors. Subtle patterns or solid shirts work well to avoid an overly busy appearance. Feel free to experiment with different textures and materials to add dimension to your outfit.