If our species is humanoid, our body design work is largely done. Areas needing description are mostly those of overall size, height, mass, and fitness standards. The prevalence of facial hair on males and average cleanliness of all genders can also characterize them, but these are partly cultural, too, and result from behaviors. A species that does messy work, like farming, might become less careful about cleanliness so that their slovenly appearance characterizes them. Deciding on some bodily issues can come before or after we form an impression of world view.
Females will be different from males in usually minor ways. Human females are more feminine than males (hence the word), but we can reverse this, making the females brutish and the males delicate; this will benefit from a good reason. Do females wear jewelry or otherwise try to appear more attractive to males, or is the species too brutish for that? Is mating like humans or do females go into heat like animals? Is there a mating season? Much of this is cultural.
Standards of beauty have changed in human history, with larger woman having been seen as better bearers of children long ago, but now thinner women are all the rage, so how are the females? Is it the women who pursue the males, who must try to attract them, and if so, what affect does this have on both genders physically? Making a humanoid body different from those of humans (in more than superficial ways) involves thinking about other aspects of their lives.
Size and mass influence not just strength and endurance, but capabilities. A species with hands that are much larger or smaller than humans will have trouble wielding weapons or using tools designed for us. If they aren’t sophisticated enough to invent their own, then is someone creating these for them? Do they just steal the items? Or do they capture people with the know-how to make them, then force these slaves to do that work?
Size also affects relationships with enemies and allies. If our species is three feet tall, do they just run away from something over six feet tall, or do they swarm while attacking? Have they developed great endurance from all that running or are they just faster than everything and then good at hiding? Being encumbered by possessions makes running harder, so do they travel light? Does another, taller species protect them? Is there a flying species who knows this running species will drop everything and flee, so they follow along hoping to pick up the discarded items, like carrion birds circling a battlefield? If they’re larger than everyone, are they fearless? Is that overconfidence that can be used against them?
In SF in particular, many aliens have skin like reptiles, not only with scales but similar coloring. This should have a biological basis, such as protection from the elements or predators. Making their skin poisonous is another option, which can introduce some cultural issues. They’d need to avoid touching humans, for example, and might be wearing gloves or other gear to protect others.
If our species is not humanoid, basing it on an animal can help realize its body and avoid something unintentionally silly. Gigantism is an option but is arguably the least interesting because it’s a run of the mill creature except for its size. If we also modify our analogue, this is more attractive.
Combining features of animals and humanoids is benefited by having some understanding why an animal has a feature so we can decide if it makes sense for our species. If our species has a tail, what do they use it for? A weapon is a good answer; that suggests protecting their rear but that they might also be a predator and prey. Is the tail is long enough to strike forward? Can that tail sting? How venomous is it? How fast acting is the poison? If it’s designed to use against those with weapon skills and not just animals, a poison would be fast acting to neutralize a threat quickly, even if it sedates instead of kills. All of this is true of poisonous teeth and claws.
Research every feature to see why it exists. Why do turtles have shells? Why don’t snakes have legs? Tails are often for climbing, but clearly that’s not true for a horse. Horns and tusks are used for fighting and even digging in dirt. Coloring may be for camouflage or warning. Wings are obviously for flight, but there are flightless birds, so understand why that happens before inventing one; they typically exist on an island that has no predators, so creating a flightless bird that lives amid many predators doesn’t make sense unless it has developed another way of surviving, one that rendered their wings less important. Not only will a little research turn up useful info to make our creations better, but it can give us other ideas and enrich our life as we understand the world around us more.