Fabric: Make sure you do your research and select a fabric that can be placed in the microwave. 100% cotton is usually the best bet for the heat pack, but a separate cover could be whatever you like.
Filling: Common fillings are wheat and uncooked long grain rice. I've used rice myself and found it to keep the heat long and you can always find it in your local supermarket.
Method Firstly, decide on the size and shape of your pack, this may change depending on what part of the body you want to use it. For example, a skinny curved sausage shape is great to go round the neck and heat the shoulders, whereas a miniature rectangular pillow shape is better suited for the lower back, abdomen and feet.
Once you know your size and style, cut your fabric to size and, right sides together, sew together leaving an opening big enough to fill. It is a good idea to reinforce the seam with an additional zigzag stitch.
Turn your fabric right way out and fill with your chosen filling. Fold back the edges and sew the opening shut. Zigzag a few times for extra strength. Your heat pack is now ready for use.
Making a Cover A cover is the best way to jazz up and personalise your heat pack using whatever fabric you choose as the cover. It should be put on after the pack has been heated in the microwave.
Covers not only keep your pack warmer for longer, but their removal means it's easy to keep your pack clean. All you have to do is pull the cover off and throw it in the wash.
The easiest cover is to create is a 'pillowcase' for your heatpack.
Sew a second bag much in the same way as you made your pack. Instead create an opening big enough to easily get your pack in and out. Velcro can be added to the opening to ensure your pack doesn't slip out. Alternatively, you can make your cover like a traditional pillowcase with extra folds of fabric enclosing your pack.
The internet is a great place to find patterns for making pillowcases if you are unsure how to go about sewing one. Or you could just wrap your pack in an old towel.
Instructions for Use
So you've made your heat pack and want to put it to use.
Place the bag in the microwave and heat for a few minutes. The exact time will depend on the size of your bag. It'll take a little trial and error. If in doubt, heat it a minute at a time until you work out how long.
Never leave your heat pack unattended whilst heating. There is the possibility the filling may smoke or catch alight. A good habit is to place a mug of water in the microwave with the pack or moisten your pack lightly before heating.
Also a new pack will take longer to heat than a pack that has been used a lot. So if your pack takes 3 minutes to heat at first, after several uses it may only take 2 minutes. My own heat pack's heating time has dropped from 4 minutes to about 2 and a half.
A warning, heatpacks should not be used as bed warmers as the heat can build up beneath blankets and sometimes catch alight. If you want to keep your toes warm in the night, put on a pair of socks or invest in a hot water bottle.
Now that you can make a basic heatpack, get creative and try your hand at different shapes. This website has created a gecko pack that sits nicely on the shoulder.
Another idea is to create a scented heat pack. Try adding dried herbs or flowers, such as lavender, to create an aromatic pack. For those who aren't very good at sewing, a cheat's heat pack can be created by simply filling a sock and tying a knot to keep the filler inside. The internet offers many sites for creating homemade heat packs.
These heat packs can also be used as cold packs, by putting them in the freezer. However, it's a good idea to restrict a pack to one use or the other, not both. So make two, throw one in the freezer ready for use and set the other aside ready for heating.