This is really easy:

1. Pick a template from gtemplates.org;
2. Select the download link and save it on your computer;
3. After this you have an archive (.ace or .rar file) which you have to unpack using a specialized software (like Winace or Winrar) because it is compressed/archived;
4. Unpacking gives you 3 main files:
   

• 1 main .psd template you can edit in Photoshop;
• 1 template preview which you can see with your image viewer;
• and maybe helpful text font or files/tools might be included if I consider it's needed.
• etc.
  

Using the template is itself simple again. You can open the .psd file with Adobe Photoshop and follow the instructions inside, or if there aren't any instructions then select the text with the type tool and replace it with what you want. Make sure you selected the layer you want to edit from the layers panel on the right. [More Help Here...]

GO AHEAD AND PICK YOUR TEMPLATE FREE FROM GTEMPLATES.ORG