In the pony generator at the bottom-left of the screen, click the colors on the left to change the pony's color (you can click the arrows to see more color options) and the categories at the top-right corner to see all the options to dress the pony.
At the bottom of the pony generator, click the arrows to flip the pony, the hanger to reset the pony's appearance, and the wand to generate a random pony look.
When you're ready, drag the pony to the field.
Click the three icons at the right of the screen to change the background. Clicking the stars icon below them will clear all objects from the field.
Once you've chosen a background, click the folders at the top of the screen to open the different categories of decoration, which you can select and drag on.