The ACK is a really fun and simple tool to use, and beause it is so simple I'm plowed through my fair shar of adventures that were less than stellar. It's way to easy to just throw together a few bricks, add some monsters and traps and hit the Publish button.
The problem here is that many people simply don't think about what they want to create before they start creating, and so you'll end up with a mish-mash of conflicting themes and ideas that doesn't make much sense. I'm not saying they can't be fun to rush through, but if you want to create something that people will remember and recommend to their friends you have to do a little more work.

In this guide I'll share with you how I prep and plan my adventures, as well as a few tips that might serve you well. I'm not saying this is the best or only way, but it's a method that's served me well in everything from Fight the Dragon to RPG Maker 95 and tabletop RPGs.

The first part of creating an adventure start long before booting up your PC or MAC. Sit down with a pad of paper and a pencil and start thinking about what you can't to create.

The first thing to write down is the setting of the adventure. This should be a single word or sentence, something along the lines of "Adandoned mine", "Enchanted forest" or "The ruined city of the Dragon Lords".

One thing I highly recommend is creating a backstory for the adventure, giving you the details of what happened before the player enter the picture. It might not be revealed to the player, but it will give you a deeper understanding of the place you'll create.
I recently created an adventure I called "Lethridge Mines". Here's the backstory I created for that adventure:



From this short story you can deduce several things:
- The adventure will have two different parts: A mine and a tomb, connected together.
- The final boss will be three Liches (Necromancers).
- The mine has been abandoned for some time, attaracting natural critters.

Nowhere in the adventure that I published have I written down any of this, but it puts me in the right frame of mine, and gives me a clear mental image of what I want to convey.

Next I jot down a few details about what kind of monsters I might meet in there, as well as some rough description of what the place will look like. If you can remember it, feel free to write down some thematic props that you might want to use.

The next step should be to draw a map of the adventure. This does NOT have to be a masterpiece. Just a rough outline with wobbly lines will do the trick, as long as it gives you a clear idea of what the place will look like. Don't ever try to draw the map pixel-perfect, there's no need for such excessive details.

Look your map over and add things like gates, checkpoints and other pieces of detail. If there's a feature like a drop down make a note of it on the map.
Once you've created your map you can either refine it with neater lines, or jump right into the ACK. I'll leave that up to you. Personally I prefer to add more and more details to my map, not only because I enjoy drawing maps, but also because it makes the place come more alive in my mind.

Here's the map I drew for The Lethridge Mines. I started out with light pencil lines, then I drew over them with some markers, and finally added some small color details. I don't always do that, but it was a slow day at work. ;)

Finally!
It's time to do the thing you wanted to do all along; fooling around in the ACK.
Start creating the level as faithfully to the map you drew and the mental image you've made up through the story and setting.

Here's a few tips on what to do and don't do when making the adventure. Some are obvious and others might not be.

Spellcheck your game extensively
In the description of the adventure and every NPC you include, you absolutely must spellcheck. A typo or odd grammar will throw many people off your game, and it cost you so little to do. This goes double if your primary language is something other than English. Alternatively get a friend that's anal-retentive about typos to playtest it.

Playtest thoroughly
Speaking of playtesting, this is something you absolutely have to do. Play and test till you're utterly sick of your game. Test with all classes. Test using a controller and test using a keyboard/mouse setup. Invite a few friends to test it for you. Go on a hunt for errors and try to break your game.
Then fix whatever you found.

Use patrols
I don't know how many adventures I've played where the enemies just stand around, waiting for you. Boooo-ring! Make use of the patrol feature and make the enemies move around. Give them different patterns. Make some go in circles while others a figure eight, some back and forth.
This will create some unpredictability for the players, and will make them seem more random than they actually are.
Just to throw a spanner into the work you might add a Wandering monster or two as well. Just beware that they can open unlocked doors.



Stick to enemies that make sense
You're deep in the Orc Fortress fighting orks, goblins and the occational minotaur, when all of a sudden you're attacked by a pair of wraiths. What the heck? Why are they there? Don't they pose a threat to the orcs?
Make sure you have an "ecology" that makes sense, and stick to it. Adding every kind of monster you can willy-nilly just because you CAN is not good level design, it's just lazyness.

Plan out your checkpoints
Each checkpoint you add make the game that much easier for the player. Not only does it give you an extra life/heart, but it also heals you up (even while fighting enemies). Use as few checkpoints as you can, preferably no more than 2 or 3, depending of the size of the map.
Also, place your checkpoints strategically. In "The Lethridge Mies" I added the final checkpoint on a high plateau. The players had to jump down to the bossfight, thus preventing them from using it to heal during the fight.

Be wary of jumping puzzles
A few people like jumping puzzles, but most of us find them either tedious, annoying or frustrating. If you HAVE to have a tough jumping puzzle in your game, make sure it's an optional thing that lead to more loot, but that won't be necessary to complete the adventure.
One or two small mandatory jumps is fine, but no more than that.