Remember to name the stage something that lets people know what it is, and give appropriate music to what you're planning to make.
Have a plan in mind, and remember stage size.
Small stages are good for quick, fast paced KO's.
Medium stages allow the average person to have a balance in action and peace.
Large stages are all about grinding up your opponent to the triple digit damage, and sending them a strong move!

You must have 4 horizontal blocks for a stage to work!
There is a limit to how much you can put in!

Now think about an actual game. Safe spots and surprise attacks are two things I think of.

Safe spots should have a balance of danger and safety.
Safe Spot Example

This safe spot (the igloo) is hard to get knocked out of since you are in a enclosed area, and there is one safe area. This area will be fought over with more people in the game. If you fall in the spikes, you could get trapped, requiring you to go all the way back.

Now look back at the igloo. See the solid block at the very left? You can only recover on a normal block. Remember that!

Now look at the top of the igloo. You can't spawn on the top due to the spikes!

Now here is a new stage, also mixing safe spots with dangerous ones. Unsturdy Bridge


The middle can force you into some spikes and if you get closed, it may take awhile to get back out! The sides, however, have a non-solid platform covering it, allowing you to jump up and attack!

There is another way to surprise attack but it involves springs. Since DataWizard has not added the spring, the FALL BLOCK will count as one. We will be using 3-space-wide springs. Fighters Den

The springs allow for a surprise attack at recovering people!

STAGE TEST

You will test your stage with Mario. For walls that people are NOT supposed to be able to jump over or walls people SHOULD be able to jump over, use this:

Have Mario Double Jump (but not triple jump) to test a wall that should be able to be jumped over. If he can not make it by 2 jumps, make the wall smaller.

Have Mario Jump thrice and imagine a fourth. If this "fourth jump" can make it over the wall, make it higher.
A better idea is to make the stage and test with Sonic. He, from what I know, jumps the highest in the game.

Remember Wall Jumps - some characters can do it, others can't. Mario can. Wall Jumps should be kept on mind!

RECREATED STAGES
Ever have a stage you REALLY REALLY REALLY SUPER DUPER UBER wanted in brawl, but it just didn't make it? You can recreate it! Test every platform for solidity, find its size, everything. You can probably do it easy, just takes practice.

Pieces
There are blocks in Brawl. These blocks can be stairs, ramps or... Blocks.
Platforms are like blocks you can duck through.
Special Parts are big, annoying, and take space a little go's a LONG way.
The Function parts take up a bit of space and also go a long way.

Extra Parts

Every 5 stages created, up to 15, unlockes new parts! I'll leave these for you to find out.

Flip/Enlarge
You can flip and enlarge things, different for each controller.
Flipped springs make then green!
You can have 2 space wide platforms...
HUGE blocks...
Long conveyor belts...
This is very useful.
If you are created a platform, use bigger pieces, instead of the smaller ones. It actually cuts a bit off the space needed! e.g.