Book cake - how do I make the edges look like pages?

[shoppingbagsundereyes]

I'm in the middle of making ds' Harry Potter birthday cake. It's going to be an open spell book with things like Harry's glasses and a snitch made out of icing sitting on top of the pages of the book. So far I have a book shaped cake ( thanks to dh who is much better at cutting than me) which I've covered with chocolate buttercream and brown sugar paste. I plan to put a rectangle of white sugar paste to be the pages. When I look at these cakes on the internet the sides of the cake look like the leaves of a book. Any idea how to do this?

I like a long clean ruler pressed in to the edge(where the white would be) as many times as can fit? And then a very thin paintbrush with pale gray food colouring just swished along about two thirds of the way on each end of each side of pages iyswim?

Also please make my birthday cake...

Buy a cheap comb, snap it to the depth of the cake and rake it along the icing to give straight even lines.

Oh that's even better...

Great ideas thank you. Am up to my eyeballs in birthday cake. Ds' bday was last weekend. Cake required for family happy birthday to you etc. Party this Sunday. Fancy cake required. Dd's birthday is Wednesday. New cake required even though there will be leftovers from ds' cake because she's already got a complex about being the last to have a birthday. Dd's party next Saturday. Fancy cake required. 4 cakes in two weeks and it'll be me who eats the majority as the others aren't even that bothered about cake!