GIMP and Inkscape are free, open-source graphics software that run on the Mac OS, Windows and Linux systems. Inkscape has powerful vector graphic tools that let digital illustrators draw images that are scalable, smooth and easily edited. This is the perfect kind of image that can be used to print on a T-shirt. But the file formats supported by Inkscape are limited, and most T-shirt printers don't accept these files. Fortunately, GIMP can convert the Inkscape file to an image the T-shirt printer can use.
Decide on a T-shirt printer, and get details about file formats they can accept, and the image size and resolution required. Many online vendors will have this information on their help or support section, or you can call or e-mail the printer manufacturer and ask.
Run Inkscape, then click on the "File" menu and select "Document Properties." This will allow you to edit the size of the empty document that's already open. Set the image size in the section that says "Custom size" by changing the width and height. Set the units using the drop-down menu; they may be pixels ("px") or standard English or metric measurements, according to the specifications of your T-shirt printer.
Use the vector tools in Inkscape to draw the vector image for the T-shirt design. If you are not at all familiar with the use of vector drawing tools, look for online tutorials that explain their use. The main difference between vectors and bitmap drawing is the ability to edit the paths, or lines, with precision and smoothness, instead of editing the bitmap to create a grid of pixels that make up the picture. You can draw shapes, starting with circles and rectangles, and then edit them by moving the handles around. You can also draw paths using the path tool.
Save the file in Inkscape as an Inscape SVG file. Make a note of where you saved the file on your computer. At this point you can close Inkscape if you want.
Run GIMP and open the SVG file you created above. You can alter the image using GIMP, if you want to, but the main purpose of using GIMP is to convert the file to a format the T-shirt printer can use, which is probably JPG or PNG or PDF.
Click on the "File" menu in GIMP, and select "Save as." Click on the drop-down menu near the bottom of the "Save Image" dialogue box, and choose the image format preferred for your T-shirt . Click on the "Save" button to save the file. This is the file you will send to the T-shirt printer.