Why the outlines are ready for you
Copying these outlines and contours pixel by pixel would take forever, and it doesn't teach you much. What matters are the two rules behind them. Once you know those, you can make as many bushes as you want. The more the better.
Rule 1: the outline is a messy circle

Start with a circle or a slightly flat ellipse, depending on the bush and the style of your game. Then take the pixels of the outline and push them out of place, on purpose. Make jaggies. In the end it looks a bit like a lion's head.
Rule 2: leaves are half-round shapes

The contours look complicated, but they're just lots of half-round shapes, one per leaf. Don't be afraid to be messy. It's nature, nothing about it has to be exact.
Step 3: Color

This bush has 12 colors: highlight tones, a mid-tone, shades for shading, and three browns for the branches. Start with the mid-tone and fill every segment. In the course I then recolor all the black lines to dark blue in one go with Aseprite's Replace Color (Shift+R). The top lines get a slightly lighter blue than the bottom ones, because the light comes from the top left.
Step 4: Shading

The light comes from the top left, so the bottom right of the bush goes into shade. Fill those areas with the shading tone, and sprinkle a few darker pixels into the corners at the bottom to enrich the texture.
Then the important part: every leaf covers the area under it from the light. So whatever is under a leaf gets shaded. And the top of each leaf gets lit, in the next step. Under the leaf: shadow. On top of the leaf: light.
Step 5: Lighting

Look for the tops of the leaves and put the darkest highlight there. Then take the next lighter highlight and use it on a smaller area, inside the first one. Every next shade covers less, and the very lightest goes on just a few pixels. Keep the bottom of the bush quiet, since it doesn't get much light.
On a bush like this you go back and forth between shading and lighting. You'll keep finding spots that need a bit more shadow. That's normal.
Step 6: Details

Finish with a few branches peeking out between the leaves. They use three browns: a mid-tone for the branch and two darker ones around it for depth. Branches lower in the bush get less light, so give them the darker brown.
And here's a bonus: the crown of a pixel art tree is often just a bush like this one, on top of a trunk.