To use Starbucks' second cup of half-price coffee fast, you can follow the following steps:

1. Open Starbucks App and enter the coffee fast page.

2. Choose the drink you want to drink and add it to the shopping bag.

3. on the shopping bag page, select the time to pick up the order. You can choose to pick up the bill immediately, or you can choose the time to pick up the bill according to your distance from the store.

4. click submit at the bottom right to complete the order.

Please note that the second half-price event may be limited by time and quantity. More preferential information can be found in Starbucks App or official website.