Actions speak louder than words. It isn't enough to say what you feel, your actions must support what you say. Seems you under stand this. Many people don't and get angry when their words don't get the friendly response they wish for. It's because there actions make their words seem hollow.
Start small. It has been a while, as you say, so don't go big and try to sustain big. That can be overwhelming. Start small, and keep it small. When they start reciprocationg, just let it go naturally from there. You will know what you are comfortable with, so when you get there, you won't have any question as to the next stage. A casual dinner out, as Baked suggested, is a good start. You can take the time then to point out a thing or two they did during your childhood that still makes you smile when you think about it. Or, if you feel nervous, just let them set the direction of conversation. They are your parents; they are likely to do that. Something they bring up might give you an opportunity to express yourself in a way you are comfortable doing. What ever you do, follow it up. That's where the action comes to support your words. That does not mean do it right then, that means something during the next few weeks or couple months. One of them have a birthday comming up? Anniversary?