That's one way to approach it, Riviet, but then you could just end up short-changing yourself because talking about the preperation for the journey is quite... well, boring. Plus you want to make some pertinent points about the journey ("themes", as such) and there wouldn't be much scope for that if you focused on the tools for the journey's preperation (though it certainly is a point to make).
Okay, so when I did this text I first considered it on its own terms - that is, a journey per se, not just imaginative or physical. I would suggest you tease out some 'themes' from the text (ie, what are this text's main concerns - companionship, etc). Not only will it help you make sense of the text, but you also will have things to compartmentalise your technqiues/quotes within (that is, you could have several themes up your sleeve to match differet essay questions - in fact, past exam questions are a great source of 'themes'). Chichi, you mentioned a couple of things -
Discovery: I suggest you treat this as what *you* the reader discover about the journey, simply because in the extract in the stimulus booklet, the journey hasn't actually begun!
Obstacle: Well this one's relatively easy because one could argue that the whole extract talks about the obstacles to the journey - eg. Toad's enthusiasm ("straddling and expanding himself") vs. the Rat ("most mistrustful"), who is almost the antithesis of the journey. There are plenty, plenty more quotes in there to highlight this point; throw in a few technqiues and you're almost there. Bút don't forget, it's not enough to just say okay there's an obstacle to this journey. So what? You *must* indicate *what* that obstacle signifies in this particular journey; lead to something bigger - for instance you could say here that Rat's attempts to hinder Toad signify that journeys are fueled by competetion, the need to express personal supremacy over one's peers. Obviously that's but one of many examples you could come up with.
Change: this doesn't have to be addressed explicitly - I know it can often be easiest to think of it as a formula (discovery + obstacle + change, or something), but I would be suggest (for this text, at least) to avoid tackling the notion of "change" head-on. Talk instead about the subtleties of character (evident by Grahame's characterisation - these are the techniques) that impact on the journey (even the very inception of the journey, as in this text).
Hope that helps...