To purchase by drop shipping on Alibaba 1688 mobile app, you can follow the steps below:

1. Log in to Alibaba 1688 mobile app and enter the "My" page.

2. Click the "Publish Purchase" button and fill in the detailed information of the goods you want to purchase.

3. When filling in the requirements, you need to pay attention to selecting the product category and brand, and provide the most accurate specifications, quality standards, quantity, delivery and transaction methods and other related requirements and information.

4. After publishing the purchase, you can search by keywords or categories to find products that meet your needs and collect seller information.

5. After selecting a suitable seller, send them an inquiry to indicate your purchasing intention, specific plan, requirements and price range, etc.

6. According to the quotation and delivery time in different situations, you can choose one or several suitable suppliers to determine the order and payment method. At the same time, you should understand relevant national policies, taxes, quality inspections, and other matters, and carefully evaluate the supplier's creditworthiness and trade reputation and other factors to avoid risks and protect your own rights and interests as much as possible.