Ooo, how lovely. That was also one of my fave books as a child.
I guess the main thing to ask is how small?
I think trellis above the walls is a good start - I would start growing a mixture of things up this, like jasmine, honeysuckle, tree peonies, maybe even grape vines (depending on where you are and which direction it faces).
Then, I would get some graph paper and plot it out. Curved lines always look better than straight. What I would do, is plot a curved path through the middle, round to a spot at the back, where I would put in a bench and maybe a small water feature, or a bird table/bird bath. I would do the path in old red bricks.
Then I would build a trellis along the contours of the path and start growing climbing roses over it, so that you can't see where the path goes from the house, and you can't see the bench from the start.
Then just start adding flower/shrub beds - maybe making the path meander along them and have taller plants at the back, going down to shorter at the front.
I would then seed the path's sides with aubretia and seed the path with thyme, which doesn't mind being walked on, but gives a lovely smell.
Add in some pots as well, here and there for interest and maybe a small herb garden as well, also in pots if short on space.
Good luck!