Everyone at some point have had the thought of making a better gadget or device than what is already on the market, and some do start creating circuits if only just to learn how electronics work.
The design process is quite simple and similar to many other project structures.

• Determining the task to be accomplished: Since every circuit is created for a purpose, be it educational, entertainment or practical, the purpose of the circuit to be designed is the first thing that should e determined.
  Depending on the complexity of the task, the project can be subdivided into individual functions that will work together to accomplish said task.
  
• Selecting components: Now that you have the functionality figured out, it's time to pick the right and necessary components to build the circuit.
  At this point it is still in the design stage, no components are bought just yet, only the datasheets and specifications of the components are required to design the circuit or subcircuit.
  This step is critical, since all the tolerances and characteristics of the working project must be considered.
  Will the circuit be used in heat? Is it expected to resist mechanical stress (as in falling, shaking, etc)? The components chosen need to fit these characteristics and also the electrical needs of the project, such as current draw, which if the circuit is to be battery powered is a major design issue.
  
• Schematic Design: In this stage the electronic circuits are designed with their corresponding schematic diagrams.
  This is used as documentation and a guide for implementation, since the design in the schematic is the one that will ultimately will be made into an actual circuit board.
  This is one of the most challenging steps of electronics design, since many kinds of analysis need to be made in order for the circuit to work as intended.
  All those analysis require a certain level of technical knowledge and experience since sometimes even with all analysis correct, experience can help you find situations that could potentially damage or make the circuit malfunction.
  
• Circuit Board Design: Now that the functional part the circuit has been designed and represented with the schematic, the next step is to design the circuit board where the actual components will be soldered.
  There are many automated tools to create circuit boards with a schematic.
  Manual design is very difficult, and with big projects with many components it could become a nearly impossible task, especially if space is an issue.
  
• Prototyping, Testing and Corrections: Another very important aspect of circuit design is prototyping and testing.
  Analysis and simulation sometimes gives erroneous or inaccurate due to external circumstances that may have been overlook during analysis such as mains hum, RF interference or other events created inside the circuit such as thermal noise, inductive and capacitive coupling and many others.
  Prototyping and testing allows you to find all those hidden characteristics of the circuit, so as to incorporate in the design before massive production.
  All this helps reduce costs as it is easier to discard one unsuccessful prototype than it is to withdraw a whole shipment that may contain up to millions of units.
  

All of the steps can be revisited as needed, since the function or characteristics of the circuit may change in the middle of the design, or the proposed design is way too complex or would take a lot of money or time to create or design.