I thought I would start with something that I use on a daily basis and which has pretty much become a necessity for me.
To start it should be known that I have pretty large feet (size 16 in the US) and due to this size I also tend to experience discomfort in my feet when not properly supported. Having larger feet it can be difficult to find shoe inserts in my size, often times they run only to a size 13 and in my shoes slide around and leave a sharp edge towards the toes. My solution was to create my own inserts from cheap readily available materials which I could size however I wanted. This is my second time creating them so I have had several years to try them out and have found they work quite well for me.
Step 1: Materials
I did not try myself due to lack of the tool but if available I bet a hot knife/wire would work very well for this. Just be sure to avoid any volatiles given off from this. Be sure to do in a well ventilated area.
Step 2: Getting Started
Making a template from scratch was a bit of trial and error. I used my foot for the template but if you're replacing a stock insole then that would also made a good starting point for figuring out size and shape. Try a template out and then test fit it in the shoe. I pretty much eye-balled any changes I wanted from there and by the second cutout I had something that was a good fit. I opt for a tighter fit in the shoe to prevent slipping.
Once you have your template, mark your lines on the mat in whatever pattern you feel will get you the most usage of material. Then its just a matter of cutting out with your tool of choice. The knife is more accurate to my template but the rotary tool made for much smoother edges. Its really up to whatever your comfortable with. Either way, always remember to cut away from your body parts, for safety. Feel free to rotate the mat as necessary for optimal positioning.
One thing to remember, which I actually forgot this time. Be sure to flip your template for half of your cut outs. Even though the cushions can be flipped for right or left, its smart to make sure you have matching surfaces on both feet so you need to make opposites. My mat had one side smooth and the other in a rough texture. If you forget like me then you will have to do a second mat with the opposite orientation, or live with mismatched inserts in each foot (no thanks).
Step 3: A Few Extra Things.
Please be aware that these may not hold up as well as a store bought support and will not give the proper support that could be had from the store. These are a cheap alternative and not a complete replacement for those with real need for arch support etc. In my case the shoes I wear have excellent arch support so the mat material conforms to this and arch support is no problem for me. They will wear over time as seen in my picture, when you feel they aren't performing as well as you would like, that's when its time to switch them out for the next pair.
They can be worn in either direction depending on whats most comfortable for you. I usually like the flat side up for ease of sliding my foot in but sometimes the rough side feels almost therapeutic, maybe stimulating blood flow or something. Whatever you like.
Also use caution when first using them. As they are so thick they may take some getting used to. I know I always feel a bit off with even the very small increase in height. Tying my shoes extra tight seems to help me feel more secure.