What you will need:
Fabric (remnants from old projects are great)
If you plan to use them in your garden and as science experiments for your kids then get these at your local garden center. The crystals are smaller than the ones you get from the craft store. These are what I used for my neck cooler.
Here is an example. I hydrated 1 teaspoon of crystals in water. You can see here that the one teaspoon became about a cup and a half when hydrated. They probably could have taken on more water.
This one is for kids. So I started with the size of a 4 years old neck, 10 inches. I added 2 inches to account for the swelling of the crystals. I then added another 10 inches for the tie. I am adding snaps to the ends of these or big buttons. If you want to be able to tie them then I would add at least another 6 inches to the length. So I cut my fabric to 22 inches by 6 inches. To be able to tie them you want overall size to be 28 in by 6 inches. (Add at least 6 inches to the length for adult neck coolers).
In this picture I have folded a piece of fabric that is 18 inches by 22 inches. This will be enough for 3 neck ties. I folded it up so that I could make one short cut. For me this makes it easier to get a long straight cut. Be sure to fold it so that roll you are cutting is 18 inches long. Use a straight edge and cut at the 6 in. mark. Now do this at the 12 in. mark. You now have 3 pieces that are 6 in. by 22 in.
Then you are going to sew starting at one corner and make a diagonal line. Like the above picture shows. You don't have to measure. Just start at the corner and come in as far as you want. Do this with all of your tubes. Now that you have one diagonal sewn, fold your tube in half and match up the edges. Using a pin mark where your diagonal stops. This will give you a reference for sewing the 2nd diagonal so that both sides look the same.Here you can see the top diagonal that I have already sewn and the other side that I have marked where I need to sew to with a pin.
This isn't as tricky as it sounds. Start at the very corner just like you did for the other diagonals. Now keep a line of sight from the needle (or the center of the pressure foot) to the pin. This will give you matching angles on the finished edges.
When you have your project right side out. Push the corners out. I used my scissors. I put them through the opening and pushed them into the corner and down along that diagonal seam to get the other corner. Do the same with the other side. If you like to iron, do so now. I don't so I didn't. ;) I finger pressed the seams.
Here I have folded my cooler in half. I want the channel where I will put the crystals to be 12 inches long. So I have marked 6in in from the fold. Do this with both sides. Now sew down that 6in mark.
Add 1/2 teaspoon of the water crystals to the center channel.
Now carefully top stitch the seam edge to close off your opening. I top stitched along both edges and down on side. You can do both the long sides, but I like it with out too.
Here you can see that I have added snaps. When adding snaps one side will have the back of the snap facing up and the other side will have the front (snap) facing up. If you don't have a tool to put on snaps then a large button would also be great.
This is what they look like snapped. It may not look very bit but I can assure you this will fit a child's neck it will also fit my neck (which measured 13 inches to a 4 years old that measured 10 inches)
To hydrate your neck cooler simply fill a sink with luke warm water (the crystals soak up warm water faster) and leave them there for about 20 to 30 minutes. Here you can see the front one has been hydrated, while the pink one in the back has not. If you lay them out to dry they will go back to the original size till you soak them again.
If you want to clean your neck cooler, do it when the crystals are dry. Hand wash with mild soap and a little bit of water and then rinse. lay out or hand to dry. You don't want the crystals to soak up soapy water. Not sure what this would do with the crystals but I would think that the soap residue could irritate sensitive skin.
Another idea for these coolers. Why not put them in the freezer and pull them out when you need something for a bump or a bruise. I haven't tried putting them in the freezer yet but I assume they would be like those freezer packs that come in a heavy duty plastic bag.