TIP FOR CREATING ART IN PHOTOSHOP.
All Photoshop (pixel files) should be created using the following starter guide, and then provided to the separator in the following as well.
1, Layered, (no background color). You can create it in layers with a background shirt color, and for your art approvals, you can flatten with the background. You can even have a merged (art file) when sending the final art out to your separator, but it must not contain anything behind it. (Not FLATTENED). All files are best if you provide it in layers.
2, RGB color mode. (Don't EVER work in and send files out in CMYK). If, by chance, you get that special occasion where you really are printing CMYK, then let the separator do the CMYK conversion (if) you are having them separate it for true CMYK printing. There are steps (color preferences) to proper cmyk conversions from RGB and must be done by someone with experience in that area. When you (or anyone) converts a file from RGB, to CMYK. That conversion is embedded into and locked into the file with whatever color preferences you have set up...and most likely, they are not specifically set up for proper cmyk printing on apparel.
3, At print size desired (such as 11" wide) x however tall but no more than max for your press is needed.
4, at a minimum of 300ppi (file resolution) from the start of creation.
Meaning, do not use a small file that has a low resolution and add it into a file that has a high resolution. That doesn't work. Also, do not then scale it up to max size...and do not just add pixels. There is a specific procedure for increasing sizes and resolutions and even then, certain things need to be done to the art file to clean up some areas.
One tip for importing vector elements into a photoshop (raster or pixel file) is that the master (starting work file) in Photoshop is to be to print size...and at 300 resolutions minimum. (I use 600). When you have that photoshop file open, and then (copy and place) from a vector program into Photoshop. There is a point found (right before committing the paste) by hitting ENTER,...and you have that box with the X in it, HOLD. Here, is where you can re-size the element to fit more closely (to the size you need it). You then also (uncheck) the ANTI-ALIASING...and then you do the PASTE. For separations, and clean crisp art, ANTI_ALIASING is not your friend. It leaves the art with fuzzy/blurred edges. If you were to create a test file at a resolution of 72 pixels in photoshop with ANTI_ALIASING off you will see the jagged edges very clearly. The jagged edges are only due to the files very low resolution, but it is very crisp and "clean" with no fuzzy edges. If it were a higher rez like 300 or more ppi, (pixels per square inch), then the jagged edges would still be there, but they would be very small and not noticeable when printing.