I recently did this. It was harder than I thought it would be because I had a lot of respect for
the owner of the company and the recruiter after interviewing with them. I was not looking forward to the call, but, in my opinion, a phone call is the best way to handle it. You just gotta suck it up and do it. They were not happy with me, but I think they respected that I was upfront about it when I called and gave them the reasons for backing out. I felt a big weight off my shoulders after the call. I left a lot on the table with the other company, but I don't regret it. It sounds like you already know what you want to do.