These are some general guidelines for making wallpapers, which I myself follow whenever I make one. I think they're useful so I thought it would be good if other people take a look at them. I don't think I'm the greatest waller on Earth, but these are tips written by a waller for newbies and veterans alike.
Let's get started then, shall we?

PART I: THE BASICS

Definition - what exactly is a "waller"?
Simple, a waller is a person who makes wallpapers. Wallpapers are images designed for desktop use only. There are many types of resolutions; you can find some of them here.
Generally, wallers make wallpapers for free and share them with other fans of the image they create/represent.

Get to know your graphics program
Be it Photoshop, Corel Draw, GIMP or *insert name here*, knowing your program is crucial. You can't do much if you don't know your way around the filters or tools. Therefore, before creating a wallpaper, mess around with the options and practice a bit. When you think you're ready to do something "professional", arm yourself with patience, inspiration, a cup of coffee and get to work!

Done already?
Some people work on wallpapers pretty fast and they do a good job too. But most spend about 10 minutes; the result is often of low quality. That is why I recommend spending at least half an hour on it. If it helps, draw a sketch of what you have in mind. I spend at least 1 hour on each one, mainly because I start off with an idea and end up with something different! However, if you like what you've come up with in a short time, good for you, you work fast :)

Viewing your work
After your wallpaper is done or even during its making, you should put it on your desktop to see how it looks. Having an idea of how it looks on someone's desktop is also good for spotting mistakes, if any, and fixing them. Remember, you make a wallpaper for yourself first and then for others. You must like how it looks on your desktop!
There are some things to take into consideration:
- the taskbar which covers a bit of the lower part - this is a good place to put your signature. It's there and it won't bother anyone since it'll be covered up.
- the icons. I have all my icons on the left but others have icons all over the screen. Try to leave a space for these by adding less elements on the sides and concentrate on the middle (not too much though or it'll look strange, having everything in the middle and nothing on the sides)

Whose is this?
I strongly advise you to place your signature on every wallpaper you make. There are a lot of thieves out there, and the only proof you have you're not one of them is that your signature is on the wallpaper you made. But signatures can be erased. So this is where the original file comes in. Always keep the source file. This is the strongest proof you can have against a thief who claims they made the wallpaper. I recommend including your name/user name and the date of the wallpaper (the month and year are enough, you can add the date too if you like). If the wallpaper features a character that does not belong to you (you didn't draw it/make it up), include the anime/creator to whom it belongs.
Another important issue is where you place your signature. While the best is on the bottom, where it will be covered by the task bar and won't be disturbing, you can blend it in with the text on the wallpaper. If you place the title on the wallpaper, you can add the signature below it, with smaller letters. Also, if you have a site/account on a site where you host your wallpapers, a link to it on the wallpaper won't hurt ^_~

Now that we've gone through the basics, let's move on to the more serious stuff...