This instructable is for realistic extraterrestrial design. Star wars/trek have mostly got it wrong- too much like earth. This instructable is for creating things like the daggerwrist or skewer from Wayne Barlowe's Expedition. Remember the number one rule: Don't make things too much like earth
Step 1: Designing the World
This step speaks for itself. Just come up with details that could effect life- heat, geography, climate, gravity, air composition, and so on. Remember that this is what the lifeforms are based on; if you give the planet a low gravity there's going to be a lot of flying things.
Step 2: Food Chain
There must be an established food chain for any ecosystem to work. this is just common sense.
Step 3: Kingdoms of Life
You don't need to limit yourself to earth's animals, plants, fungi, and single cell organisms- try some things that are a combination of these things- like the plents of the Sagan 4 project. It really doesn't matter as long as you have a basis for a viable food chain.
Step 4: Animals- Sensors
Animals don't need the same senses earth creatures have. In Expedition the is almost an entirely planet wide lack of eyes, you can have vibration sensors, thermal pits, echolocation, or even bioelectrical sensors like those of sharks. You shouldn't, however, use telepathic energy- it is unscientific and these should be scientifically plausible worlds.
Step 5: Animals- Body Plan
Remember the first rule. We're not the pinnacle of evolution- not all planets will evolve like us. There is, however, some basic rules for life, creatures, at a certain point, will evolve something like a spine, there will be brains and hearts and blood- although not necessarily lungs.
Try some weird but plausible things to- tails with claws on them, natural air cannons, lungs in the form of a tube, etc.
Step 6: Animals- Feeding
There's no need to have jaws when there's so many interesting mouth parts out there. In Expedition the primary method of eating is liquivorious (animals pumping food full of enzymes then drinking them) and some creatures even eat through feeding grooves that act like cheese graters. Just remember that there's multiple ways to eat food.
Step 7: Animals- Reproduction
You need a way to keep your planet alive so you need to reproduce. Macroscopic animals can't do this asexually without dangerously cutting down on genetic diversity and you need to establish how they start the next generation. You can make different genders or make hermaphroditic creatures. Give them mating behaviors like changing colors or just have one sex have plates or whatnot.
Step 8: Planimals (Also Known As Plents)
Planimals are just plants/animals. They exhibit behaviors of both. They may move or may be stationary but they will have a heart, blood, a form of lung, and be able to use photo synthesis
Step 9: Plants
Plants still work on the same principles that they use on earth. One possible adaptation is gas bladders that allow them to rise above a forest canopy to get light.
There will still be things like grass, moss, seaweed, trees, and cacti (maybe not, depending on the planet's stage of development).
Seed dispersal will also be similar to earth except in a few cases such as ballooning fruit.
Leaf colors will be decided by the colors filtered through the atmosphere- we don't get a lot of green light so our plants don't absorb it as much.
Step 10: Fungi
These are necessary decomposers. They allow the dead life to return to the earth.
Step 11: Single Cell Organisms/Colonies
These aren't necessary to mention but it's always interesting to have slime molds.
Step 12: Civilized Life
DON'T MAKE IT TOO HUMANOID!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Remember the #1 rule and make differences. They only necessary similarity is a way to manipulate the environment. They don't need homes either- we do because we're so weak compared to other animals.
They will probably be mid-range predators; having the necessary brains to catch prey and the need to avoid predators.
Step 13: Just a Note
I'm lazy and this is just the basics. I'll be updating it later so keep checking.