Timing IGT is fairly simple, first thing that you want to do is do your actual run of the game and get your RTA. From there you can open up live split and start inserting your segment times for IGT. How you do this is by taking the video of your run and then you put it into a video editing software and time each load. There are two frames specifically you are looking for, so you can reliably find the beginning and end of each load. The first is the frame that either the level name appears (Summoners Rock, Fetid Forest, etc.) or when it says "please wait". The frame in which you stop timing is the first frame when the screen goes completely black. Here is an imgur album with the frames I am talking about. https://imgur.com/a/S46FI

Now to the second part of the IGT timing. We are then going to take the accumulated load times in a segment and subtract them from the RTA segment time. Once we have that time we are on easy street. Just plug that into your IGT segments in LiveSplit under the Game Time tab and once all segments are in, it will give you a total time, and that is your IGT. Here is what it should look like, RTA on the left IGT on the right. https://imgur.com/nlts7jq Then you should save your splits and upload them to splits.io

Good luck with your runs everybody :D