This paper presents a sketch-based platform for intuitively modeling a 3D apparel product around a 3D human model with pre-defined features. After briefly introducing the nonmanifold data structure utilized in our system, three techniques are reviewed to obtain a feature human model from a laser scanner, a size table, or photographs. With the feature human model, the user can interactively draw several 2D freeform strokes on the screen to construct a 3D garment by specifying either the boundary of a garment on the surface of a human body (for tight clothes), or the profile of a garment template related to the feature human model (for relatively loose clothes). Our platform supports two modification operations using 2D strokes to modify the constructed garment in the 3D space; they are mesh extrusion and mesh cutting. Finally, after cutting the modified garment into several pattern components, each component is flattened into 2D patterns for the garment manufacturing processes. Our system provides an intuitive and efficient platform to create and modify apparel product patterns directly in the 3D space; and the geometry coverage includes closed surfaces, open surfaces, and more complex non-manifold objects.