This tutorial will help you especially the beginner to know how splines use in interior and exterior furniture designing. First of all check that your chair has correct dimensional. Always start the modeling in top view, for chair modeling, take rectangle in a top view and zoom in. click anywhere in the view port and you will see that there will be rectangle in modifier option. Right click on rectangle and then click on rectangle spline. In modifier there will be segment option select that option. Press one segment and with Ctrl key select the other segment as well.
Another option divide will help you in creating more vertices. Just right click on the object and there will be option of corner select that mode. Now select move tool and give them shape as much you required. When you will see in prospective view this will be in solid rectangular shape. With help of filet command you can set the vertices and edges. Now draw a line for back and seat and enable the render and view port. Select its vertex and use filet command to give them round shape. You can see the round shape by doing zoom. For seat, extrude the modifier from modifier list and adjust it as much you want. It will be final result.