How? you learn it like everything else - at school and by experience.
Take a course at a technical college, learn a trade in the electronics industry, read books on the subject, you know, study it.
Electronics and circuit design is not something you can learn here from scratch, you must have some general electronics knowledge, gathered at school, in the industry and elsewhere and ask here about a specific problem that still puzzles you.
Start reading - its an interesting subject if you have the right mind for it :wink:
Klaus