Function Analysis is a good way to analyse how existing products work. Testing the product is the solution to conduct such analysis: it can be done within a team where some people test, other record or take notes and photos of the different steps.
You can construct a process tree like the example picture where you focus on input and output energies, materials and signals and each action conducted one after the other or simultaneously.
Another way is to create a 4 columns table: with first the main functions, then the sub-functions related, the supporting functions and finally the desirables. This solution will enable a decomposition of the problem and thus it can be easier to find out solutions. In the example of a mixer, the main function is to "comminute/blend food", one sub-function could be "avoiding food poisoning" and thus a supporting function would be "provide easy cleanability". In this case, the founded diserable is to "provide a better assembly system".
In order to design products for all, meaning also disabled people, you can try to put yourself in their place and conduct the function analysis. This will probably open a new whole of problems and desirables.