Download the meme template to your computer so you can open it with a graphics program. You can draw directly onto the meme or copy and paste your work onto it later.
Download and install a free graphics application to your Mac if you don't have one already; free apps suitable for this purpose include GIMP, Seashore and Inkscape. Not all of these programs are as full-featured as a boxed retail product like Adobe Photoshop, but not all memes ask you to publish completed and polished work. Install what works best for your particular needs and budget.
Open the meme in your graphics program. If you have a graphics tablet, use it to draw directly onto the image. You can also create a new image document to draw in if that is less distracting or more convenient. If you don't have a tablet, use physical mediums like colored pencils and paper, then scan your images to your computer. Crop and move the scanned images to the meme document. Alternately, you can print the meme, although many are quite large vertically, so this may be challenging.
Save your completed meme to your desktop to upload it to deviantART. One of the better formats to use when saving your image is PNG, as it prevents loss of quality without making the file size overly large.