Before you create 'Coupon Codes', please ensure that you have enabled the 'Coupon Code Field'.
Once this tick box is enabled, to create 'Coupons', from the store dashboard go to:
> Coupons > All Coupons

Click 'Add New' at the top of the page to create a coupon:

Within the 'General' tab you can 'Name', 'Code', 'Activate' and decide how much 'Discount' you'd like to offer on your 'Coupons':

The 'Restrictions' tab offers additional criteria based settings which restricts how 'Coupons' are used:
*Underneath each of these settings is a description which informs you of what they each entail - please read them carefully*

'Applies To' is the final heading tab on this page which allows you to choose which 'Products' or 'Categories' you'd like your coupon/s to target. 'Shipping' costs can also be discounted with 'Coupons' via the tick-boxes below:
*Beside each of these settings is a description which informs you of what they each entail - please read them carefully*

Click 'Products' or 'Categories' to attach the coupon to 1 or multiple 'Products' or 'Categories':

(*Don't forget to click 'Publish' after making alterations*)
After publishing the coupon, it will be listed in:
> Coupons > All Coupons
Hovering over an individual coupon will allow you to 'Edit' (re-edit), 'Quick Edit' and 'Bin' it:

In 'Quick Edit' you are able to allow only strict access to the coupon via 'Password' or via a 'Private' link.
Additionally, over to the right, you can alter the 'Status' of the coupon. You can either change this to 'Published', 'Pending Review' or 'Draft'. *Only 'Published Coupons' are able to be used by your customers*:

(*Don't forget to click 'Update' after making alterations*)