These are a very simple project that a beginner could make very easily. I usually make mine so the the finished project is 10" by 6." Another idea I do is to make a cover to go over the bag. If I do make a cover then the fabric for the pillow itself is not important as it wont be seen and can be any old fabric. If you don't make a case for the pillow then try a fleece or flannel for the pillow as it is nice and soft.
Usually for someone who is making these only once, the inside of the pillow is filled with rice. People who sell these often use flax seed as it is a better choice. Flax seed has oil in it instead of water so when you heat it the heat is retained longer. A good read to see the difference is on this site. Whichever you use, rice or flax, you will need a good 2 1/2 cups of it for this project. You can also throw in a 1/3c dry lavender flowers and or a few drops of lavender essential oil.

First cut out two rectangles of your fabric of choice. They need to measure 10 1/2 by 6 1/2. This leaves the 1/4" for a seem allowance and will leave the finished project at 10x6. Place right sides together and sew up three sides of the rectangle. The fourth side you sew all but 2 1/2 inches, leaving an opening to put the rice in through.
Turn your pillow right side out. Pour in your rice or flax seed. Add any lavender or essential oils you may want to add. With a needle and thread stitch up the opening and you are finished!