Asset art has started to become more popular on this wiki, and I have to say most of it is kind of disappointing.
So here's a guide on how to create decent asset art.
1. Resize or recolor your parts. This can make your parts look different from how they originally were, and you can form shapes that you need to add to your plant.
2. Create your own parts if needed. I usually do this as assets are pretty limited.
3. Use layer styles. Use inner glows to create a border. (less size, more hardness)
4. If you're making a variant of a plant, just use its original art and edit it from there. It's easier.
5. Merge parts together when needed. This is kind of hard to explain, so you'll have to learn this yourself.
6. Don't make your plants look like obvious asset abominations pieced together. Make it seem real.
7. Use proper shading, using tools that darken or brighten your parts.
8. No blurry parts. Just no.
9. Use free distort to fit costumes/accesories if needed. If it gets too blurry, then undo.
A product using Step 2. (Cucumber part made from scratch)
An example of Step 5. (Head is merged to the berry)
An example of Step 3. (Cap uses inner glow)