1. Make your design in “raster based” applications like CorelDraw & Adobe Illustrator, they produce very sharp output. You can also use Adobe Photoshop and other “pixel based” software but make sure you use at least 300 dpi resolution.
2. All images to be used (especially from internet) must be in high-resolution to attain maximum print quality. Zoom the image at 100%, if you can see jagged edges do not use it unless you can trace or re-draw it.
3. Manual drawings should be traced first in Photoshop, CorelDraw or Illustrator.
4. Don’t design over a seam, pocket & zipper. It creates print inconsistency because these are thicker portions of the garment. Here’s an example: