To use a 3D text mockup PSD, you would generally follow these steps:

1. Find and download a 3D text mockup PSD file that suits your design needs.
2. Open the PSD file in Adobe Photoshop.
3. Identify the layers or smart objects within the PSD file that are intended for your text input.
4. Double-click on the smart object layer or text layer to open a new window or panel.
5. In the new window or panel, edit the text with your desired words, phrases, or typography.
6. Customize the text properties such as font, size, color, and other relevant attributes.
7. Save the changes and close the smart object window or text editing panel.
8. The mockup PSD file will now display your text in the 3D format, incorporating the predefined 3D effects.
9. Customize any additional elements, such as background, lighting, or texture, if desired.
10. Finally, save your modified PSD file or export it as a different file format, such as JPEG or PNG, as needed.