First thing you should do is copy the skin folder of the skin you are using as a starting point and rename it before you do any customization. Otherwise if you leave it weith its current name it wil be overwritten the next time you upgadre mojoportal and you will lose your customizations. So, its very important to give it a unique name to avoid any conflicts with existing or future skins included in mojoportal.
Next, you should read the documentation about skinning thoroughly.
Unfortunately I have no step by step instructions for you. Its a trial and error process even for those of us who have a good understanding of HTML and CSS. We make an educated guess about how to change the css to gradually change the look as we want it. Make small changes and refresh the browser to see if it worked. A good understanding of Html and especially CSS helps us understand what to change to achieve the desired affect.
Hope it helps,