The mood

The general mood of an adventure is of great importance. Although some people think that it is predetermined by the game, which is why Warhammer Fantasy RP is always associated with dark fantasy and AD&D with dungeon bashing. This is false. The mood is determined by the GM and the adventure. Just because the rulebook reads of grim world, does not mean of how the adventures will look like.

Take the computer game Discworld as an example. The first part was very close to the novels, very funny and full of absurdities. Then take a look at Discworld:Noir. Totally different. If you like AD&D and want to play in a dark fantasy environment, just do it. It is not necessary to buy a new rulebook.

The general mood has a big impact on the game itself. When it takes place in a so called dark fantasy environment, the players are often much less likely to trust anyone. When you set it in a world full of powerful magic and chivalry, they are much more self-confident.

What you prefer is up to you, but it is necessary to think about the mood before the game starts. It is plainly stupid to create an adventure full of dragons, balrogs and sorcerers and then make them so powerful, that no PC can stand up against them.

Also remember that the PCs have to be aware of the general mood. Make it clear that they cannot have a "wish" every now and then or you end up with dead PC and angry players.

The Plot

The plot is difficult to make. A good rule is to keep it simple. Not that the players should be able to foresee everything. There is a huge difference between simplicity and blatancy. Shakespeare's plays are very simple: right versus wrong, good versus evil. Everyone can understand them. That is what makes them popular. You can hardly find a movie today that is not following a very simple plotline. The general theme needs to be simple. No one, not
even you, is able to present an adventure with three dozens people intriguing against each other and having different aims.

Although the general theme should be simple, it is difficult to make it appear complex. You can simply add a few red herrings, but this will just distract from the plot. Take for example a very simple plot, a knight that wants to save the princess from the dragon. Quite simple. Now add a third party, that is not caught in this good versus evil battle, but whose aims partly contradict the aims of the dragon and the knight. This makes it more complex, without adding much complexity. Also remember that the simpler the plot, the easier it is to follow. Otherwise every now and then the players need to be briefed about the identity of different persons.
Most amateur adventures are much too complex. They add everything they have in mind, hundreds of subplots and all their favourite characters. This is okay with a novel, but a no-no for an adventure, unless you are very good in which case you should think about selling it. If you have written an adventure count every main character and every main aim. Now compare these numbers with that of a good professional adventure (e.g. Harlequin for Shadowrun or Horror on the Orient Express for Call of Cthulhu). If you exceed the number of the professional adventure by more that five, reconsider the adventure.

A simple way to come up with a plot is to steal it. Movies or books give enough material to exploit. The difficulty however is that it should not be too obvious. Use allusions and general themes, never ever simply copy a plot or a character. There are too many "Gandalphs", "Gandafs" or "Kandalfs" running around. If you for example create or flesh out a religion or cult, it is good to take a look at the various churches and cults in our world, it is bad and a sign of a lack of imagination just to copy the roman-catholic church. Don't do that unless you are really unable to come up with something new.

Also avoid extreme turns in your story. Everything changing from one side to the other is okay, but difficult. The players may feel cheated. Plot changes should never ever put the PCs at a disadvantage. That does not mean that their situation should not change, otherwise it would not be a turn. What I mean is that the players should not feel cornered.

Also avoid to bring every idea you have. Less is often more. Subplots and red herrings are good, but they are a distraction. The players not only have to think about their own characters, but also of the NPCs, while at the same time lacking your superior knowledge.

Use stereotypes and clichés. They make your life much easier. A port tavern is easier to imagine if it is greasy and run down, but hard if it has marble floors and room service. As long as you stick to clichés, you do not need a justification. Every player I know of wants to find a weak spot in an adventure, give them little chance to find one. But do not overuse clichés.
Otherwise the adventure is too blatant.

An advise I can give you is to keep the plot open. It should be finished and everyone should know that the job is done. But it is a good idea to keep some threads open. The players will have interest in them, unless they are brain dead, and, if you wish, you can add further adventures with one or more thread running through all of them.

Do not try to make a funny plot. Even if you find that something is hilarious, it is not sure if the players agree. A great example is Discworld. The world has its absurdities, but it is not outright funny. Neither are the characters. The fun develops more out of absurd situation or
totally strange reactions towards certain circumstances. Never ever try to force jokes, it will not be funny. Also playing a funny adventure is often quite difficult and tiring. If you are not very good at jokes in roleplaying, either avoid it at all cost or use very small doses of jokes.

Try to avoid passages that need to be read out. Although this is tempting, because it is convenient for the GM and the author can present his ideas like he wants to and not through the mouth of someone else, I think it is bad roleplaying. And bad roleplaying should not be supported by adventures or modules. If the GM has too read more than a few sentences or lines, the game gets boring. If you want to hear people read, run for parliament.

Your first adventure does not need to be epic drama. Start with small ones. They are much easier and give you better idea of what happens. You have more control about the plot.

Do not create an adventure that requires a certain class of PC or a certain skill. An adventure should be possible to play by almost any combination of skills and classes. Otherwise you restrict the possible audience too much or force the GM to introduce a NPC that supports the players actively (see below).

The villains

The villains are the spice in your soup. They transmit your ideas and the adventure. Craft them carefully, but do not hesitate to kill them if necessary. The worst villains are the Dr.Moriaty types. No matter how often and how hard you kill them, they reappear. These villains should be left for superhero comics and bad movies.

A good villain is one upon which death your players have a feeling as if they also lost something. He should be an integral part of the environment. A convincing background is good. It is vital to have an idea of the general motivations and psychological traits of the main villains, as wells as of the good guys. Do not try to put the main characters into a corselet. It is better to have an open character, where you just have an idea what he will do in certain circumstances, than a closed shop that moves on definite tracks.

Development is probably the key term for all NPCs, probably with the exceptions of the absolute minor characters. Especially the main character are more convincing if they develop through the adventure, particularly in a longer adventure or campaign. A self-confident arch-villain that could kill a village with a quick flick of his wrist can become unsure about himself,
after the PCs have made a few severe blows.

It is not a bad idea to abuse a minor character as a on-going joke. I remember a Shadowrun campaign, where the PCs have knocked out a ticket inspector. Later in that adventure they wanted to raid a flat in some run-down area of Seattle. Unfortunately they got a clue wrong, forced a door open and in front of them was a person with a familiar face, a broken nose
and a black eye. Of course they hit him real hard with a trenchgun and later, when one of the PCs had to go to the hospital, that person reappeared, this time paralysed from the neck downwards. I use them encounters not only for entertainment, but also to provoke surprise and astonishment in my players. It is also a good way to illustrate
consequences.