To run ILs, you will first need to access the level select screen. This can be done by reading this short guide: https://www.speedrun.com/tom_and_jerry_gbc/guide/q3ulk

After selecting the level you want to run, you will select "Player 1" mode on the selection screen.

Time starts when you select "Player 1." You can use any timing program, even the timer app on your phone.

Time ends when Jerry hits the "Exit" sign at the end of the stage and the level finished music plays.

That's it. ILs can not be ran through normal gameplay mode though. They must be done from the level select method due to differences in timing.