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First you run the seam tape on one side, put sides together and heat until you can see that it has bonded. I followed the directions on the package, but had to add a heating time due to the thickness of the placemats. It is important that you check to see that the tape has bonded before moving to the next side. It will look like it has melted and "glued" the seam together.
Repeat this process with the other two sides, leaving one short side open.
The last step is to stuff the pillow and seal the end with the same process of seam tape and heat.
Ta Da, you have a pillow!!! I added a satin bow to match my bedding. I think I even surprised myself that this worked so well.
The only tip I have is to be sure you check the bond on your seams before moving on to the next side. I found this to be a great bond that is holding the seams very well.