He means cut the edges off of the picture, so you just see the vehicle.
Here, let me give you some preview making tips, it's how I make my previews.
Open the editor, change your preferences, set the background to black or grey; anything but blue. You can change it back to blue after you make the preview, I understand if blue makes it easier for you to see the model as you work, but it looks bad in previews.
Open model, choose an angle you think looks good. Take a screenshot.
Open your screenshot in paint, or paste your screenshot into paint. Type any information you want to be in the preview around the model. Use selection box, drag it around the part of the preview you want. Ctrl+C to copy. Paste it into a new paint, and save it. If you want more angles, then paste multiple screenshots, and drag the pieces together.