How can I remove iOS 8 and get back iOS 7?

I recently downloaded iOS 8 without researching much... I have an iPhone 4s and noticed it was fairly laggy after i had downloaded the new software. I looked online and many websites said that the 4s could not handle iOS 8. I had backed up my phone to iTunes before downloading iOS 8. I tried to erase my phone and restore it, but it continues to give me iOS 8. Some websites suggested downloading the software from the internet but i would prefer to avoid that as i don't know which websites are legit. I just want iOS 7.1.2 back! Please help! Thanks.