image > image size
This will give you the basic method, but there are nuances. If you are starting out with a high resolution image, you might want to resample the image to the native resolution the printer. For example, many minilabs using Fuji or Noritsu printers use 300 pixels per inch. In this case, the best resampling method would be bicubic sharper, as suggested by the drop down menu in the Photoshop resampling box. If you are using a camera that does not have a 3:2 image ratio, you would have to crop the image.
Then you would probably want to sharpen the image. A small print can use more sharpening than a large one. For this sized print, you might try the unsharp mask with a radius of 1.2 – 2 and an amount of about 80-150%, which is a larger radius than you would use for a 16 by 24 inch print. The optimal settings for glossy or matte paper surfaces are also slightly different. Photokit Sharpener automates the process, but for quick results on small prints, the above settings might suffice.