How do I design in Photoshop for Darkroom Booth

How do I design in Photoshop for Darkroom Booth

Darkroom Booth  |  Documentation  |  Free Sample Template
                                                                                                                                                                              
You will want to design the template at size 300dpi. If you are designing a 2x6 strip the resolution would be 600x1800.
Here is a sample PSD to help you get started.