I have played some premade adventures with my group, and want to try and make my own. Is there any good "guide" how to do that?

Cheers

I am at work right now so I can't really post up as much as I want to, but this is a topic that's important to me as well- I can, hopefully, provide some advice on it when I get back home.

I do want to say that talking about your ideas with other GMs is always a good way to get started: come up with a planet you want to set your adventure on first off that sounds interesting to you if you want something concrete to start with- then come up with a problem related.

There's the guide to investigations stickied on this very forum, a few topics above this one- and book of judgement has the plot generator which in my opinion provides a lot of inspiration. Furthermore, the internet is rife with DMing guides that (though very abstract) offer ways, methods and suggestions on setting up a plot from scratch [Which often go along the lines- create X amount of Npcs in a certain setting and let them loose or other ways of weaving things together.]

Once I get home, I will post something more extensive and more interesting to read!

I don't know of any guides on how to create an adventure, but here's my attempt at one. Just a warning though, creating adventures usually takes a lot more time than it does to play through them. You may spend days working on an adventure that gets played through in one session.

Anyway, this is how I make my adventures(in order of steps)

1. Theme/Genre: Start with the theme of the adventure. Think of what type of adventure you want to run before anything else.
Action - Many variations, but generally combat heavy
Exploration - dungeon crawls or epic journeys through exotic untouched(by man) locales
Investigation - The token Dark Heresy theme, detailed in the sticky thread and also chapter 4 in The Book of Judgement
Intrigue - social encounters with more emphasis on role-play than the other themes,
Horror - designed to scare the players, not just the characters. With so many sub-genres, this theme could be an entire topic by itself
Espionage - deep undercover missions, infiltration, huge emphasis on stealth

2. Setting: Now that we have a theme, we need a setting that will compliment the theme of the adventure.
An espionage mission set on a desert world doesn't fit too well. Just like an exploration mission wouldn't fit in a place already discovered.
I prefer to think of the setting first, and then try to find a planet that fits it, but that's personal preference.

3. Background: Once the theme and setting have been established, we should think of what the story of the adventure is. This is what happened before the players got involved. The players may never come to know these details, but it is important to establish it for yourself. Detail the goals of what should happen if the players never get involved or if they fail. Which leads us to the next step.

4. Primary Objective: This is directly related to the theme, what should the players accomplish in this adventure?
Examples:
Capture/Kill the antagonist
Survive/escape
Retrieve an item or information/Rescue someone
Explore a new area
Solve mystery

5. Plot: How will the players go about accomplishing this goal? Here's some skeleton plot structures to choose from. These can be combined, as it's possible to have more than one plot in an adventure.
Site-based progression: Players journey to different areas and face encounters as they progress
Gather elements: Players progress by collecting information, items, clues, evidence, allies, or any other element necessary
Unlock objective: Players find their primary objective immediately, but need to accomplish a series of events in order to access the primary objective
Series of deadly battles: Players are constantly set upon by memorable dynamic combat encounters that are all life-or-death situations
Distractions: Players are in a dynamic setting, constantly being distracted from their primary goal

6. Climax: The climax is not our last step, but it will conclude the adventure in some fashion. Some basic tips to keep in mind for the final moment:
Foreshadow - Make sure that the climax is foreshadowed throughout the adventure
Raise the stakes - When the players finally reach the climax, the consequences for failure must be catastrophic.
Epic Effects - The climax should take place in a location that's bigger, more dangerous, more exciting than anything before it. You have an unlimited special effects budget, make it epic!
Dynamic Elements - There needs to be more to the climax than static dice rolls. The players should feel encouraged to do something different every chance they can. Essentially, surprise the players with a multitude of different challenges as the climax progresses.

7. Details: Be prepared with your adventure. Add detailed descriptions of locations, people, items, etc. Make note of any special rules you are going to use, any creatures/enemies, what the personalities and strategies are of NPCs, what kind of sights and sounds will the players experience. Create events that will add flavor to the setting, add scenery that will enhance the theme. Make maps, player handouts, props. Have your adventure come to life!