将新设计集成到旧的rails应用程序中

I was asked to integrate new designs into an old app, but the designs are completely different from what there is now. Due to the functionalities already there in the app, I had to integrate the design into it. Starting with the homepage, I replaced all the html.erb files but there is a bootstrap theme which is consistent in the background. I also have to integrate new js into the app. Can anyone please guide a newbie through this? Been at it since 2 days and I don't even know where I am going wrong. Working with rails 3.2.13. Any suggestions or advice for the same would be very much welcome too. Thanks in advance.I was asked to integrate new designs into an ol