You could actually build a level that incorporates your sketches directly.
Maybe I'll try something like that in future, but for the time being I'd rather use existing tilesets.
Some level of planning is a good thing, but you are getting carried away here.
[Nifflas] recommends to do this kind of thing directly in the level editor.
As soon as I started working with the editor, I realised the folly of having been so specific on paper. For one thing, I can browse through tilesets and choose what to use, which alters the ideas completely.
Anyway, here's a look at the opening screen and the screen directly to the left (The best idea I could think of to force the player to go right was that Juni lived in a lakeside house (perhaps in part fed by the waterfall that will appear later?)). I'd welcome any criticism about backgrounds or colour combinations or anything like that.
(http://img203.imageshack.us/img203/8404/scrnc.jpg)
Work is going slower than anticipated, because a) I'm finding out the limitations of working with only 2 tilesets at once, and b) I've been going to actual picnics (all research, I swear!). Anyway, I've redesigned the opening screen slightly to accommodate a change I wanted on the next screen along, which is a transition to the other area you can see below. I've also slightly redesigned the general layout: rather than starting in a city, I've decided Juni lives just outside the city but must travel through it to get to the valley to meet her friend.
(http://img706.imageshack.us/img706/6665/unledhg.png)
(http://img24.imageshack.us/img24/8508/unledch.png)
Juni at the foot of the mountain. She's so close, but so far!
I've been learning how to work with the editor and layers quite fast - the way I build the screens now is very different to how I made the first couple. I've also found confidence from knowing about Bernie, a colourblind game developer, and been using tiles and backgrounds based on what they look like to me, rather than my original plan of asking people what colours this tile, or that background, was. I /think/ it's going well.
Alas, there aren't any diagonals on that tileset (Orange Grass). I'll look through and see if there's anything similar (I chose it for the tree models) but if I can't find anything better, I'm somewhat happy with it at the moment. (Edit:Maybe a change to 'Red Forest of Beauty')
One thing I'm having trouble with is a very small part of that screen - I've added a small series of jumps to blocks to reach the first ledge, but they're just floating blocks - nothing holding them up (see below). I might scrap that idea and put the climb powerup before the hill.
(http://img859.imageshack.us/img859/3166/unledsx.png)
Update: I've redesigned the mountain area (and backgrounds preceding it) based on LPChip's advice to smooth the backgrounds out. It looks a lot better, in my opinion. Thanks for the tip, LPChip! I've also decided to place the climb powerup under the tree and scrap the 'jump from floating blocks' idea.
(http://img860.imageshack.us/img860/5420/unleddl.png)