You can learn to design printables with free options like Canva, or Paint. These programs are fine for basic designs, but can be very limiting. If you would like completely custom designs, programs like Adobe Photoshop and Illustrator are excellent.
For maximum efficiency with cutting printables, consider in investing in a cutting tool like the Silhouette Cameo or Cricut Explore. Both allow you to print out any shape and cut around the printable. It takes minutes to set up the file and cut compared to hours of using scissors.