A tree is a bush on a trunk
The crown of this tree is the bush from the previous lesson. No need to draw a new one: paste the bush, put it on a layer under the trunk, and you're done with the hardest part. That's why the crown is ready in the trainer.
The trunk uses a main brown, five browns for the light and the bark texture, one dark shade for shading, and two greens for the grass.
Step 2: Outline

A trunk has three parts. A wide foundation at the bottom, about 22 pixels. A narrower body above it. And a top that spreads out again, a bit like a crown of its own, where the branches start. Move one pixel out of line on the way up so it doesn't look too straight.
Step 3: Color

Fill it with the main brown and recolor the outline. In Aseprite, Replace Color (Shift+R) does the whole outline in one go.
Step 4: Shading

The light comes from above, and the crown blocks it. So the part of the trunk right under the crown is a little darker. It's barely visible, just a few pixels. But small details like this are what separate pro work from amateur work.
Step 5: Bark and light

Here the highlight is also the bark texture. It runs like a spiral, from the top right down to the left, in little steps. Start at the top with a couple of bits, then work your way down. Inside the first layer, add a lighter brown on the top side of each band, since the top is closer to the light. A few in-between pixels here and there smooth the steps out.
Steps 6 and 7: Grass

Finish with grass around the roots in dark green. Spread it a few pixels to each side, cut some gaps into its bottom edge so it looks textured, and let a pixel or two stick out like a root under the grass. Then a few lighter green pixels on top.
By the way, the trunk is good enough to use on its own, without the crown, if your game needs a bare tree.