The specific method of purchasing installments using credit cards in Douyin is as follows:

1. Open the Douyin app and find the product you want to purchase, and click the purchase button.

2. Select credit card payment in the payment method, and select the credit card brand and bank that need to be supported.

3. Confirm the product information, select "Instalment Payment" after confirming the payment amount, and select the required number of installments and related handling fees.

4. Enter credit card related information, such as cardholder name, card number, validity period and CVV code, etc., and confirm the purchase order.

5. Next, follow the prompts on the payment page. After waiting for confirmation of the SMS verification code, you can complete the process of purchasing Douyin and paying in installments with a credit card.