beginners guide to installing themes

Hello - I have recently had ubuntu 12.04 installed and have been adjusting to operating ubuntu instead of Vista. I now want to add a theme, but I am totally confused on how to get it to work properly. I have read many threads on themes over the last three weeks, but I have not found an idiot-proof solution. I have Myunity and GnomeShell installed, and the ones I have tried to install by pasting the codes into the terminal as instructed, just don't show.

I appreciate you pro's will think this is such an easy process, but for someone, (64 years), who has used Windows for about 25 years, its really confusing. Can anyone tell me where I can find a step by step guide to installing a modern theme please? Perhaps a link that really breaks every process down with a good selection of themes for 12.04? Thanks in advance. Regards, Tony Brown.