From FreeCAD Documentation
- Make sure you have at least three edges or curves in space forming a closed contour. For example, these can be created with tools of the Draft Workbench or the Sketcher. Using three edges would create a triangular surface; four edges a quadrilateral surface.
- Optionally, curves can be drawn inside the closed contour, not necessarily touching the edges. These curves can be used to control the curvature of the resulting surface.
- Likewise, a number of vertices can be used with the same purpose to indicate where the surface must go through.
- Press the Surface filling button.
- Inside the Boundary section, press Add edge.
- Use the pointer to pick the desired edges in the 3D view; a preview of the final shape will be shown after selecting valid edges that form a closed contour.
- Optionally, go to the Curvature: non-boundary edges section, press Add edge, and pick the desired edges from the 3D view.
- Optionally, go to the Curvature: non-boundary vertices section, press Add vertex, and pick the desired vertices from the 3D view.
- Press OK to complete the operation.