Sketch projection can help in constraining the newly created sketch with existing model. Take a Sketch projection of existing model/face on a plane in which new sketch will be created. Sketch projection projects the faces/bodies on a plane and creates permanent lines/curves. To take a sketch projection, Right click on the plane >>Insert >> sketch projection >> select the face/edges >> Generate. When a new sketch is created in the same plane, it can be constrained with projected lines, curves or points. To constrain new sketch use constraint options from Sketching >> Constraints tools.