Vertices- Defining points at either end of a spline segment.
Segment- The part of a spline between two vertices.
Steps- The number of segment divisions used to represent a curve. (More steps = smoother curve)
Tangent- Handles on a Bezier Vertex, dragged to control curvature of the spline segment as it enters, and leaves the vertex.
1. In the create tab, select Splines from the drop down menu. (default) Click where it says LINE, and customize the properties of the spline you are going to create, then create the points by clicking in the viewports. (Don't use the perspective view, for lack of presicion)
2. When you're done, right click to terminate spline creation, click on the modify tab, and scroll down to where it says "RENDERABLE" check it on, and define a value for the thickness of the shape. These changes won't be visible in the viewports, but if you render it you can see what it will look like in the final image.
3. To edit an existing spline (add, subtract, smooth, sharpen or edit locations of vertices), in the modify tab click 'Sub-Object' to use the array of functions available for editing your shape. Click 'Sub-Object' again to get back to the normal editing mode.
A) To change the type of vertex you are editing, right click on the point and choose from Smooth, Corner, Bezier or Bezier Corner. Bezier vertices can be controlled with a tangent.
B) If your spline looks too jagged, increase the Steps, or check on 'Adaptive'.