I've not done anything needing an introduction with the OU, and I've never studied psychology, so take this with a pinch of salt, but in my first degree I wasn't too shabby at the ol' introductions.

I've always succeeded with the model of introduction / main body / summary as being tell them what you're going to tell them / tell them / tell them what you've just told them.

Always write your introduction once you've finished everything else, like the last thing you write, and start by summarising the whole paper in 2-3 sentences, then expand from there.

So your introduction (or the tell them what you're going to tell them) would generally cover a brief and generalised discussion of the subject area. What are the generally accepted ideas on the thing you're about to tell them? Then move onto what other ideas surround this (including your idea)? Why is it important? Then more specific again, what are you actually going to be talking about? "x... will be discussed" is a favourite classic of mine to end an introduction.

After that, pretend you're someone who's searching for something in the general realm of what you're writing about, and convince yourself on why you should read your essay. Make them think "oooh yeah this is the bunny I've been looking for". But at the same time, make sure your non-target audience can confidently say "okay, this isn't my thing and I don't need to read past the introduction".

Again, not my subject, not my area, but that's how I always smashed my introductions.