The process here is very simple. Please download a Free app called: Bulk Discounts (developed by Shopify), which will add the option of generating unique coupon codes for your clients. This option will be added to the 'Discounts' page at your Shopify's back-office.
At the discounts page you need to select the amount of coupon codes you wish to generate, we recommend on starting with 1000 codes.
Click the 'Export' button.
Copy the codes (click Ctrl+A), and paste them in a txt file (notepad), and click the upload button.