What do I need

There are some things you need.
First you need a printer. A good ink jet printer with 360 dpi is more than enough. Make sure that you have a good paper though. Single sided glossy 360 dpi paper is what you need.
A painting program. If you can lay your hands on Photoshop grab it. If you can't get GIMP, a free painting program for Linux with many cool filters.
You might want a scanner and a drawing program (like Freehand or Corel Draw). These are optional.

Beginning

You might find it useful to know your painting program. You should know how to work with multiple layers.

Now get a ruler and look how high and wide your banner pole is. Then draw the overall shape either direct in the painting program or in a separate drawing program.

Then start with the background. Don't make a to complicate background. Try a gradient, a filling pattern (Gimp has a lot useful of these) or the cloud filter.
I used the Photoshop cloud filter for my banners.
The banner on the left has two Nova filters in addition to a blue/black background.

The background for the chaosdwarfes army standart is made with the GIMP lava filter. The runes are made with another filter. In fact for most of the drawings I used a filter in the progress.

Pattern / Motive

Start a new layer for the foreground. You need a motive for your group. I used a biohazzard symbol for my plague monks. For the vampires I used a heart, a chalice and a kind of vampire sign like in the film Blade.
Don't go for something complicated. I tried to paint a kind of demon with a claw reaching out, but it looked awful. If you don't belive me, look at the GW regiments. They hardly use anything more complicated than a tree or an eye for a banner motive.

Writing, last effects

Start a new layer for the writing. Most banners feature the name of the regiment. The script-fu functions of GIMP come in handy here.

At last you can finish the banner with some effects. I made lightning for the vampires banners. This is very easy to archive. Just paint multiple white or bright blue zig zag lines on the background.
Next you can paint some small black spots on the banner with the airbrush tool. The middle of this spots is later removed with a scalpel to make scorched holes.