Mastering a Master Layout using Adobe Illustrator and Adobe Photoshop
It’s a great idea to combine several design files into one main file if you know what you want. Specifically, you’ll need know the fabric type you want to print on and its printable width as well as your design size. For the purposes of this tutorial, I will be working with six individual designs that will print onto one yard of our 54-inch wide cotton voile.
Step 1: Know Thy Fabric. Spoonflower fabrics vary in their printable widths. I started this project knowing that the cotton voile is our best hankie material, so I knew that my design would need to fill a yard of voile that measured 54 x 36 inches.
Step 2: Math is Good. Some simple math is required to size the design to print correctly. Our default print resolution is 150 dots per inch (dpi), and the printable width of the cotton voile is 54 inches, so I multiplied 150 dpi by 54 inches to figure out the file width of 8100 pixels. I did the same to figure the file length: 150 pixels x 36 inches = 5400 pixels. I guessed that the handkerchiefs would print three across, but 18-inch hankies (54 divided by 3) seemed more like dinner napkin size than hankie size. I decided to make them 16-inches square instead. 16 X 150 = 2400, so this means that my master file needs to measure 2400 x 5400 pixels at a resolution 150 dpi. 
Sub-Step 2.5: Math is Still Good. Now that I know the dimensions I’m working with, I can figure out how to size the individual designs within the master file. I decided on a 16-inch square because that allows for a half-inch of seam allowance all around each design. 16 inches x 3 across = 48 inches, and 52 inches – 48 inches = 4 inches to spare along the entire width. So the inch measurements went: 1 + 16 + 1 + 16 +1 + 16 + 1 = 52 inches total width. My length measurements went 1 + 16 + 1 + 16 + 1 = 35.
This left a bit of extra width and length around my 52 X 35 inch file, but I decided I would simply order the file as a centered design to avoid another partial repeat starting. I’m not much of a perfectionist.
You’ll notice that I created smart guides at every intersection of the aforementioned measurement. (Those are the blue grid lines.) You can create guides by clicking your cursor within the rulers in your workspace, then dragging out the line and dropping it at the mark you want. There is a guide dropped at the first 1-inch seam allowance, then another at the 17-inch mark (1+16=17), and then another at the 18 and 19-inch marks, and so on.
Step 3: Obey Your Master. It’s definitely worth mentioning that your designs need to be sized using the same specifications as your master layout. I also made sure that my individual designs were all in an RGB color space, and that the resolution of 150 dpi and the pixel dimensions agree. In this case, my designs needed to be 2400 pixels square (because 16 inches x 150 dpi = 2400 pixels). When I’m done designing, I always save in the native format (like .ai or .psd), and then also as a .jpg or .png.
Step 4: Come Together. It’s just a matter of importing your completed designs saved as .pngs or .jpgs into the master document now. I prefer using the “Place” function found in the File toolbar menu, but it’s possible to simply drag and drop your files into the master layout canvas. For maximum flexibility, I create a new layer for each individual design. The layout will be likely be rearranged, and keeping organized in this way takes a lot of the work out designing. One thing I’ve learned is to never cut corners and to bite the bullet when it comes time to defining layers, using color swatches, etc. It slows things down, but imagine having to untangle elements into layers later. Yikes!
Step 5: “Importer/Exporter.” (Seinfeld reference – sorry, nerd alert.) Now that all your designs have been carefully imported and placed into the 16-inch square guides, you can save the file and export the master file. The file will be large due to the sheer number of designs it contains, so it’s best to export as a .png or – more likely – a .jpg. Spoonflower’s maximum file upload size is 40MB. Again, make sure that your dpi is 150. This resolution prints well with us and will also help keep your file size down enough to upload to Spoonflower.