How fire works
Before you draw, it helps to know why fire looks the way it does. A bonfire has a heat source, and it sits at the bottom. That's where the fire starts, and the hotter something is, the lighter its color. So the bottom of the fire is the lightest part.
As the flame grows up and out, it gets a little cooler. It's still very hot, but cool enough to change color. So the edges and the tip turn deep orange and deep red. The sparks are cooler too, which is why they're darker than the heart of the fire.
That's the whole logic: find the heat source, make it the lightest, and get darker toward the edges.
Step 1: Outline
Start with an outline that looks like a water drop. That shape works for most bonfires. Add a few sparks above it, they belong to the fire too.
Don't just copy the reference. Look at it and measure by eye: how many pixels across the bottom, how many going up, where the tip starts. Measuring by eye is a really useful skill for any artist, and even more in pixel art, where every pixel counts. You don't have to be super strict, just close.
Step 2: Color
Now fill everything with one flat color, no texture yet. The fill tool does it in one click. Then recolor the outline and the sparks with the same color. The outline has done its job: we know the shape and the size, so we don't need it black anymore.
Step 3: Shading
Here it gets interesting. Take the deep red and go along the edges, since that's the coolest part of the fire. Then add some orange inside the edges, and turn the sparks orange too.
You don't have to match the reference pixel for pixel. If you want a red pixel here or there, go for it. The reference is there to guide you, not to be copied exactly.
Step 4: Highlight
The highlight comes last on purpose. Once the outer texture is done, you can see how much room is left for the heat source. Put a few pixels of light yellow at the bottom, a little taller in the middle.
Finally, the background
I like to finish with a background. It gives the fire contrast and makes the shading and the light stand out. Keep the fire on its own layer and put the background on a separate layer below it. That way you can move the fire around or change the background later.
Your turn: two more bonfires
Same palette, same four steps. Open a 26 by 26 canvas and try these two. The first one is still a water drop, but the flame bends to one side, its tongue bends too, and the heat source is taller.
The second one is your challenge. Study it before you start: how many pixels at the base, how high the flame tongues go. Look at the shading too. On the sides it's only one pixel wide, and it gets two pixels wide toward the tongue. The heat source here is small, so it doesn't take much room.
A ground flame
Same logic, stretched sideways: a burning floor or a trail of fire. The heat source is the whole bottom line, so the lightest yellow runs along the ground, and the tongues cool into orange and deep red toward their tips. Vary the height of the tongues so the row doesn't look like a fence, and throw a few sparks above.