Read Mark Sisson's book, which is convincing. It's necessary to be convinced! Then start. Of course you will fail. But because you are convinced, you will not give up. It may take a few days, or if you are like me it make take many months, but you'll get there. At first you will fail in a multitude of ways every day, and the project will seem impossibly ambitious, but by taking two steps forward and one step back you will slowly get more and more consistent. Inevitably there will be days days of complete disaster, but over time it will become hours of complete disaster, then minutes. And one day, it'll be second nature.
"Then start," I said above. But where? I personally started with cutting out grains because that resonated with me, and because I couldn't face cutting back on sugar. At some point the grains became easy. So I was ready to give up sugar, but not artificial sweeteners. As I got more educated about artificial sweeteners, I sadly learned that I had to give them up too, but that's easy to do once you're educated enough to know how bad they are for you! I added the other pieces as the urge struck me.
You will doubtless do this differently, depending on your temperament. You may like to do things all at once. You might like to go piece-by-piece. You might have a sweet tooth like me and decide to tackle that later than you otherwise would. You might decide to keep in dairy, but then find cutting it out gives you better results. You might find that 85% cocoa chocolate is a great cheat at first, but then you start to binge on it so you need to cut it out. As long as you are convinced and persistent, start wherever you are moved to, and you'll do fine. Just keep experimenting, and never say die.