From what I remember it looks like organised chaos at the border but its all quite straightforward and took us about an hour to get our papers. Like any border, don't take any photos though - someone did when we were there and had their camera confiscated.....
There are a number of "official" helpers at the border (with plastic passes round their necks) who you are expected to tip if you use. We used one and he was genuinely helpful in getting us through the paperwork, although its all doable without one.
I'd get a green card in the UK if you can. I got one from Norwich Union and it was a lot cheaper than getting it at the border, which my colleagues did (they paid 80 euros for 4 weeks I think?).
Best first night stop is Chechaouen (about 3 hours from the border I think), at the edge of the Rif mounains. Great little relaxed town - campsite on the hill above the town if you need it.
Have fun - Moroccan pistes are the business!!