*requirments
-Winterboard and open ssh installed on it.
-Some program to acess the iphone with (like winscp, iphonebrowser, cyberduck(Mac))
-A photo editing software (not obligatury but extremely usefull)
-A program capable of converting .mp3 sounds to .caff (only if you want a custom UI sound)
1.The Theme Folder
The first thing to do is to create a new folderwherever you like in your computer. Name it like the name of the theme you'd like to have. Inside it create this new folders with the exact name here:
-Icons
-UISounds (only if you'd like custom, sms, mail, calendar lock and unlock tones)
2.Wallpaper
For the wallpaper you can get any photo you like from google, from a webpage or from your own photo library. All you have to do is opening it with a photo editing software an change the picture size. In paint (at least in windows 7) you have a resize option. The sizes you have to put are 320px wide and 480px tall. Be careful with paint because you have to have the "maintain aspect ratio" option disabled and that will cause paint to strech your photo. This doesn't hapen if you use photoshop where you can go to the top bar and click image>>adjust canvas size and set the parameters. Then you just have to do a free ransformation of your image by presing ctrl + t and aumenting or reducing your image to your needs.
After you are done with that just save it as a .png file with name: Wallpaper (for behind the main menu) or LockBackground (for the lockscreen). Be careful because it is case sensitive. Save it in the main folder of your theme.
You might want to have a transparent dock so you see your entire wallpaer, for doing that download this file (right click > save target as) and place it in the main folder of your theme as well.
3.Icons
You can have anything you'd like as an icon for any application, I'll show you how. The first thing is to get an image from wherever you want for your icon. Then, as I've explained above resize it, but this time to 57px x 57px. Make sure you save them as .png files and in the icon folder. The name you have to put to the icon image is the name of the app as it appears in the homescreen. Be careful, it is case sensitive as well. You can use this method for internet shortcuts.
4.Sounds (only for sistem sounds)
The sistem sounds of the iphone are the ones for the text messages, for the email, calendar alerts or lock and unlock. You'll have to use an application to convert the audio files to caf. This is not such a big deal since iTunes itself does it. You can get any free audio editing software to cut the part if the audio that you want firs, then import it to iTunes, right click the song and select convert to aif (you might have to change iTunes preferences). When you have done that go to the directory of the fille and change its extension from .aif to .caf Then rename it to one of the following and save it on the UISounds folder:
(this method will not add new sms sounds, it will only replace the deafult ones)
sms-received1.caf
sms-received2.caf
sms-received3.caf
sms-received4.caf
sms-received5.caf
sms-received6.caf
new-mail.caf
alarm.caf
mail-sent.caf
SentMessage.caf
unlock.caf
lock.caf
*This is just a variety of the sounds aviable to change. If you want to see all of them ssh into your iPhone (explained below) and get into /System/Library/audio/UISounds.
5.Get the theme into your iPhone
To put your theme inside your phone, as I've alredy said you'll have to get an ssh program. Here I will explain the settings for Winscp (iphonebrowser doesn't need any settings because it works by USB), first of all conect to your home wireless with your iphone and click in the blue arrow near to your wifi's name. The ip that appears (usually 192.168.1.x) is the one you have to enter in the ip adress space, then enter "root" as user (without quotation marks) and "alpine" as password. choose sftp as conection type and go for it. The first time you shuld get a warning, but its completely normal, just go on and wait. When you get to the main screen look for the "var" folder in the bar in the right, then enter "stash" and then "themes.(randoom numbres and letters)" Then drag and drop the theme folder there. Go to your iphone, open winterboard and select your theme. Close winterboard and let the iPhone respring, and You're done!!
If you have any doubt feel free to comment.