ESP-01 and user friendly update of wifi credentials

I've my own arduino device that connects to my personal wifi network.

But, how can I do if I want to sell the device and let clients to setup their own credentials?

Which are the most used methods? Where to store user's wifi credentials? And, how in a friendly way?!

Where to store user's wifi credentials?

An SD card?

And, how in a friendly way?!

A PC-based application.

Ehm... A little more specific?? :) Any example?

brisu75: Ehm... A little more specific?? :) Any example?

First, is saving the data on an SD card an option? Second, what programming languages do you know? What OS are you using?