I created a single pilot operation part 135 for a company that I am currently employed for. There are a 3 other companies that are operated by our parent company so the 135 is used within our companies and not offered out to the general public even though is listed as a "on Demand" air carrier.
You have to start with having an airplane to use (you can own or have an individual that is willing to lease it to you when needed) and contact your local FSDO and providing them with a "letter of intent". From there you will be assigned a POI to help guide you through the process to create your Op's Specs that are tailered to the services you are wanting to provide. I was able to find off of FAA web site all the paperwork and examples of forms in word format and I just tailered it to fit our operation. My FAA guys were a great help for me.
You will need to think about the drug testing, AAIP, MEL's, insurance requirements (that is expensive for any charter "for hire" operation), RNAV procedures, hazardous materials.......
The good news with single pilot is there are not training requirements or training manuals or general operations manual. It is the easiest way to go. You can always add pilots and the manuals as you grow the operation.
Good luck with your endevour!
Corp-Capt