|This is sort of human. Doesn't really need|
it's own monster page.
1. Create a simple chart with details that will largely matter when describing the combat ability, size, shape, intelligence and location of the monster. The chart should be simple to read and should not skip information that will appear in the text ~ especially the special attacks!
2. Explain the chart. Some things do not need explanation; some things can be codified and then explained on another page expressly for that purpose, such as describing what a "scattered" distributiion is or what is a "wet/dry tropic" [it's coming, I swear]. It's most important to explain the special attacks, how the monster fights and the general shape of the monster (height and weight plus the picture are not enough!).
3. Describe the monster's habits. What is it doing when it is not eating player characters? When is it out and about, how does it get along with friends and family, where does it hang on Saturday nights and what rings its bell? The monster has a motivation; it needs describing past how it uses its body to kill things.
4. When appropriate, skip details about things that the players ~ and even most people in the game world ~ simply won't know about the monster. A lot of monsters are so rare, or so powerful, that what it is doing on vacation is way out of general knowledge. Drop a hint or two about the missing information, but don't feel every detail has to be hammered down.
5. Avoid getting cute with the amount of detail. It sounds fun to give the monster additional nick names or describe how a particular culture worships the monster as a god, or even to go into detail about how it gives birth to others of its kind, but in game terms this is just useless information that needs to be plowed through when the players are waiting. Describe the monster in sharp detail; the extra details can be added later, during the adventure or if the players actually witness such a birth. Those details don't need to be in the rules.
7. Your players read, so expect them to read your monster descriptions. which is one more reason not to add too much color to the description. Remind your players that your monsters are a guideline, because ...
8. Monsters have species. Just because this manticore happens to have a long tail with spikes doesn't mean every manticore does. A manticore can have a scorpion's tail, too. So when the players think they've got a monster pegged, remind them that size, body type, peculiar habits, intelligence and motivation can differ. The description is a guideline.
9. Add pertinent details once the players become familiar with a particular monster. The next monster I mean to write up is the killer frogs, or froglings, the online. Since the players already know the creature, this is a good time to include all the additional information that has come up ... but since it doesn't mean its the last time that they'll find these things, so I need to keep that monster universality in mind.
10. Every monster is dangerous. Perhaps in a very different way, perhaps obliquely, but nevertheless dangerous. Some are dangerous because the players won't see them coming, some because of some annoying ability they have and some because they appear in frightening numbers.
Why is dangerous important? Because they're monsters. They're not paper targets. They're the nasty, unpleasant, murderous side of nature and everything the players are supposed to be terrified of meeting in a dark, cold landscape. For me, I don't believe in XP farming by killing, say, rats. There are no "safe" monsters.