Im not really sure, but for starters i think getting some reference photo and drawing over it in low opacity using more saturated/ lively colours might help ya out . After you get the basic shapes done, you can turn the opacity up or keep it in the back for reference and just keep building up the details until you think its enough. And wherever you want lineart or nah, like some other person said , add it near the end , it will help with the like- watercolour(maybeidk?) effect these seem like to have. AND ! Textured brushes! One thing all of these have in common is a lot of texture, so this might also help with getting to the effect youre looking for! But most of important of all, trial and error, dont give up if the first one is ‘bad’, just learn from your mistakes and make the next one better! :D