Starting out today I will make a full post of the diagram and conception for the gels and how to attach them to your Canon 580EX and now for the 580EXII flash.
There are several ways you can do this and here is another.
You can get this printing pdf diagram for the 580Ex here and for the 580ExII (New) here.
With this print you then can cut your color gels as illustrated on the next DIY color gels next shots.
After you print the diagram onto some paper then it's time to match it with the color gel as you can see in the picture.
Cut it with a nice scissors :)
With the help of the ruler fold the upper part of the diagram with the gel underneath it. You can use a ruler behind to force it a little.
There you go. Now you got the gel shaped and ready to put into the 580EX or 580EXII.
Final product shot.
You can use different colors if you like ;)
P.S. - I also have another version that I have attached them, to the flash, with some Velcro... I leave it up to you on what will suit you best and more be more practical.
If you have any assembly shots for the Nikon let me know so I can link to them.
Stay tuned for more stuff.
P.S. - A BIG Thank you to Thomas Reufer that mention it first at dforum.net