How do I change application icons

I have a couple of software word processing applications that just show a bent upper corner on a blank sheet with the letters 'TXT' on the bottom ....for an icon in Snow Leopard.

How can I make the application (any application) display a custom icon unique to each application ... instead of the generic looking icon (described above), when I save a file.

I also am now getting a similar bent corner icon ...with XLS when I save an Excel file ...instead of the one it used to generate when I was using Tiger. The default icon in Tiger had a green colored header on the top of the default icon when you saved a file. It also had the Excel icon in the upper left corner.

I can see the default icon for Excel when I do the get info ...but it doesn't display like that when I have the file in a folder. How can I get it to display the correct icon when in icon view in a folder?

I know how to change an icon to a custom one individually ...or quantities all at once by copy and paste in Get Info window. I'm wanting to have the application 'create' the desired icon when I am finished making a new file from an application by itself ...without having to copy/paste ...to change it to the one I want.


Hope I wasn't to confusing with my questions.

 

Thanks! That helped a lot with two applications I use often: Excel and iText. They both now generate their default graphic icons when I save a file from them.

I'd still like to know if there is way to substitute the default icons that a program displays to a custom icon of my choice, if anyone can help or steer me to where to get information on doing that ...within Apple's OS ...without the need to purchase a third party app like Eyecandy.

Thanks again.