Please don't say that. It is not at all easy, and I'd advice you deconstruct some other themes before you dive in headstrong thinking it's simple and flooding the forum every time you run into a problem.
There are two different types of icons - those for native apps, and those for apps installed through the appstore. The positioning for native apps is slightly different to that of appstore apps, and some Cydia apps use the native template. I'd recommend reading
this post as it's helpful to understand how iOS icons are actually put together on the device.
To use the icons in a theme, you need to have the icon's name and the app's bundle identifier. Appinfo from Cydia is a brilliant app that gets this information for you most (not all) of the time. Folder structure, transparency, file names and capitalization are all very important when putting a theme together. I can throw something together quickly in less than an hour because I practiced for days on a personal theme, looking at lots of themes to see what they do (I recommend Glasklart, open source and very much filled out).
As for making the icons themselves, I suggest learning how to use layer styles to achieve certain effects, it comes in very handy. Try to make everything big and scale it down, it's much better than doing it the other way.
I'm not trying to discourage you, I just thought I'd let you know it's a lot of work, not just throwing an image you quickly made in a folder and onto your device. I hope you stick to it, learn some stuff and start making awesome themes
🙂