As someone who was recently broken up with, I can tell you there's no easy way - especially if there's no obvious trigger for the separation.
I think the first step always has to be making absolutely certain that you're sure this is what you want. From there, it's really up to you to choose your medium. If you think you can trust him to respect your decision, it might be easiest to do it over the phone. This way you can prepare what you want to say, read it off and if it does start to get messy, you can just hang up. Alternatively, an email might work just as well - but be sure to call/message before you send it and let him know that you're serious about everything in it and to please understand as far as possible.
Don't worry too much about the exact details of why you don't want to be with him any longer - these will only give him ways to try to argue with you, and will trouble him for a long time. Just try to explain that you're not happy, and you are ending the relationship, and it's not his fault and there's nothing he can do.
And then, honestly I think it's best to end communication, at least until you've both moved on. If you're still likely to see him in your day-to-day, then there are other ways that might be preferable, but I think really this is about as good as it gets.