[owlpower]

I don't understand why this molecule will rotate plane polarised light.

I look out for 4 different groups attached to the C atom (with CH3 group). That is the C with H and CH3 attached. I just need to decide between the remaining 2 groups.
At the 4 position (where the C=C bond is), I don't know how to assign as the carbons are at "equal intervals" between each other. I can't think of a better way to phrase this, I'm sorry!

[PdG3Precatalyst]

You shouldn't look for an atom with 4 different substituents. It is classical example of axial chirality.

[kriggy]

I wouldnt call it a "classic example" but yeah.

Try to draw the mirror image and you will see.

[owlpower]

I wouldnt call it a "classic example" but yeah.

Try to draw the mirror image and you will see.

Do you mean this? https://imgur.com/a/N0dT4CO

[kriggy]

Pretty much. Now you can see that both mirror immages are different and they cant be superimposed.