Basic models are costructed from a series of blocks of geometry either additions or parts of the geometry removed; with time and experience you will be able to visualise the stages and techniques to use to create the intended final form.
Basic models are created as solid models; the blocks of geometry used to make the model are called features (more about this later).
Each model stage or Feature is 3d; each 3d shape can be described by a base 2d sketch. For example a cube is a 3d shape, the base sketch or one side is a square; a 2d shape.
Each stage of the model geometry requires a base sketch; and the 2d sketches can be drawn on any plannar (flat) surface or plane.
The image shows a ghost outline of the final model followed by a series of cylinders being created one on top of the other. Each cylinder is added and merged with the previous geometry; the model is one piece of data and at this stage is called a "part".
Each cylider is created by using the Extrude Feature and the Feature is based on a Sketch containing a circle.
The model is not complete at this stage; it needs the chamfers and the screwdriver slot adding (or removing). The geometry is created by using the Chamfer Feature and the Cut Extrude Feature. This is explianed later on the course.
The technique used (above) will produce the correct geometry for your part but it might not be the most efficient method. The revolve technique would be better in this case.
This model is part of the course. See the Stepped Shaft tutorial blog and download the tutorial drawing from the course exercise page.