There is InSim.txt in the docs directory of LFS - it gives an overview of how it works and all the tech specs of every packet. That's definitely the first stop!
What programming language do you know, and how is your programming in general (beginner/intermediate)? There are a few frameworks available, depending on your language.
What are your end results? What kind of program would you like to make?
There are various approaches, you could use many different programming languages, but to make your life easier some guys already made some libraries for inSim that have all the required functions you will need. It would be smart to use that and for start just make an app that establishes the connection with inSim in LFS.
I can't code, but I learned to use LFSLapper, where a guy named Gui-Luron made his underlying code interact with the LFS InSim, which was then fairly easy to change, especially as most of what I was doing was really just amending and adding buttons and labels.
I would start with the explanations on how to make buttons.
The main script within 'lapper' has very good explanations of what particular parts of the script is doing, and sometimes how to change part of it to get a different result.
Many moons ago when i first started off, LFSExternal was the first library i ever got into. Working with logic and just putting things together was a blast.
While i do agree that making an insim might be a bit overwhelming for someone that hasn't done any prior programming. The chances of success are not 0.00%.
Let me put it this way, for the OP:
InSim.NET is a C# library made by DarkTimes, with contributions from the community. This library makes it much easier to handle the packets between LFS and the insim client. To get an insim up and running, you'll need at least some knowledge from C# or any other programming language. Other than what the others have said above, there's not really much else i can provide.