Hello, I have the most recent version of Blitz3D and the most recent version of the SCP CB source code.
I have no idea what file to open nor how to make custom item combinations for SCP-914.
If anyone could tell me how, link me a tutorial, or link me a video I'd be very happy.

It's pretty easy. The function is called Use914, line 9421 (Main.bb). If you are wondering how to edit what happens to the player, goto Case "914", line 7351(UpdateEvents.bb)