Hi and welcome to my first instructable.
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
For this project I want to keep the materials pretty simple. All you really need to do this is your base material and a way to cut the shape. I prefer the puzzle style cushion mats which I was able to pick up at Harbor Freight for under $10 for a 4 pack. Even with my size 16 shoe I get quite a lot out of even a single mat so make as many as you want or have time for. I also experimented with cutting with a rotary tool this time and while it was much easier than a knife and left a cleaner edge I find the knife to be easier to follow the shape.
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.