Board design Process
In general, no design spec is required. Especially since this is a hobby forum. Just jump in and start building whatever gadget you are interested in. If you want to document something, start by listing the problem you are solving. Then detail the items like physical size, power requirements, inputs, outputs, features, etc. As the list grows, you may choose to further sub-divide this into items that will be in the initial version and items that will be in a later revision.
You will begin to see the building blocks of how it will all fit together. Some blocks will be needed earlier and some later. Make a timeline for each and then map them out on a calendar or Gantt chart . Finally, start designing the first block in the timeline. Again, if it is a hobby, you do not have to make formal documents or even work on the blocks in order.
Be aware that everything will be revised. As the project evolves, you will update, change and refine all the documents and plans.
For work related stuff, it is best to follow the guidelines used by your employer. There are no standards. Ask for the documents from a similar past project and use those as a template.