When ordering food at McDonald’s via mobile phone, if you want to choose sugar-free Coke, you can follow the following steps:

1. Open the ordering software.

2. Enter "McDonald's" in the search bar and click "Enter".

3. Select the product you want in the menu bar.

4. When selecting a drink, find the "Coke" option and check "Sugar-free".

5. Confirm the order information, complete the payment, and you will get the sugar-free Coke you need.

The above steps are applicable to most McDonald's mobile ordering platforms, and the specific operations may vary depending on different software or system versions.