Colour Mode – RGB
Pixels Per Inch – 300ppi
Optimal type to save file as – TIFF
Always create and save artwork at a 1:1 ratio for print. If you make a file 20cm x 20cm it will print at that size.
You can also save files as a JPEG, a TIFF is a higher quality image format
If you find your file is very large you can reduce the size by making it 150ppi
When working in Photoshop remember to flatten the layers before saving