I don't think I could make a tutorial for icons. Paul Boyers 'Icon-A-Day' is probably the best.
I learned a lot by going through different Photoshop Tutorial sites. It was learning all the different things/effects and such I could do with PS and then applying them to icons.
As for how long it takes...for me, some of these are relatively simple and easy to do. Demonoid was a bit of challenge, but it's through experimenting and such that I learned what works best for me with certain things. They get easier as you go along. I like when someone throws out something like Demonoid. Demonoid took me a couple of hours. The 'war game' icons are challenging as well. One of my favorite was the BlacIce icon. It took me afew hours to get exactly what I wanted, and someone else could have done what I did or beter in minutes, I'm sure. My absolute fav and one I am proudest of is Folding At Home. The site that it was designed after liked it enough to consider it as their official icon. (I'm still waiting to hear) That took me a while and I wasn't sure if I liked it when I was done, even.
If you look at the first one I did for this thread, I think it was CDWave editor, you can see how I have progressed. I may not be great, but this has been a great learning tool for me, making these requested icons, and a lotta fun.
My TSWOC icons have been time consuming. So was my Shut-Up-And_Kiss-Me Icon pack. Finding a theme is easier than maintaining it through the entire set. That can be very challenging.
If there is something specific you want to know, I'd be glad to try and help all I can.
