Step 3: Preparing the files.
I'm assuming that if you have Photoshop, you have some level of comfort within the program. Even so, I will try to explain it as best I can for those who don't
With that said, open the file and make sure the guides are visible like in the picture below. If you don't see the guide lines go to the menu bar on the top (view --> show --> guides). By selecting this you should turn on the guide lines.
Next make sure the "Snap to Guides" option is selected. Again go to the top menu (view --> Snap To --> Guides).
Now your file should be ready to begin work.