How to generate gift card for customer?

You dont have to because whole process is fully automated. Below you can find description of how it works and when module generates gift cards.

Gift card generation process
Whole process is automatic and based on configuration of the module. below you can find 4 required steps:

1. Let's say that you created a "gift card product" (see how to create gift card product)
2. You created association between "gift card product"  (see association between gift card - vouchers)
3. You defined settings of voucher code (see voucher code settings)
4. You decided when you want to send voucher code (see main settings configuration)

So now, when someone will order "gift card product" - module will search for voucher association for this product, if module will find such association - it will generate voucher code with settings you configured in step 3. Module will do all of this only if order status will be the one from selected order states (for example generate gift card if order state is "payment accepted").  Module works the same also when you switch the order state manually.