Portal - Render Textures and Camera Projection Matrices

Hello!

I've just started a personal learning projection where i'm trying to replicate the portal mechanism from the game Portal. I'm using render textures but my problem is that they appear flat, almost as a TV screen. This is obviously not the result I wanted as these portals should visually look like physical holes into the space from the other portal. From what i've googled i've learned that I should move / rotate the camera according to the cameras projection matrices but to be brutally honest; i've got no idea how to even start to solve this problem. I've seen similar questions on this forum but I really can't get my head around it.

I'm not a math guy and i'm relatively new to programming so if you know how this works, then it would be incredibly helpful if you could explain how I should go about solving this problem. I'd owe you one!

Thanks for taking your time /T