First and foremost - generate multitude of images. My go-to setting is 4 batched of 4 images at euler, 20 steps. When you like the composition and general direction of some result, you can lock seed or send it to img2img to build more elaborate and detailed variations.

AI was taught on square 512x512 pictures. If you need other aspect ratio, try to still keep smaller side at 512. You can upscale the image later, or even use generated image as a base for larger image.

When AI gives you something you don't want, try to exclude it by using negative prompts. Like with positive prompt, there's a trick. You should use things that could be realistically used to describe existing picture. Hardly any picture would have caption containing "extra head", so adding it to negative prompt does almost nothing for example.

AI was taught on all kinds of images. Photos, childish scribbles, pictures, game screenshots. It can't tell which is "good" and which is "bad", you have to guide it. Be elaborate.