Step 1: Go into Cydia (Make sure you have winterboard installed, if not install now)

Step 2: Go to the "sections" tab on the bottom in cydia.

Step 3: Scroll down into till you get to themes and choose what you would like to change, preferably "themes complete".

Step 4: Find your theme and click on it

Step 5: Than in the top right hand corner, click "install", than "confirm".

Step 6: It will than install and on the bottom when it is done it will prompt you to "return to cydia". Click this.

Step 7: Exit out of cydia and go to "winterboard".

Step 8: Turn off any other themes you may have on and toggle the theme you would like. Than exit winterboard and the theme should load.

Yay your new theme is there!