I've always wonder how to do it and never figured it out. Can someone help?
This is a pretty broad question. The best I can do without actually making it for you is point you in the right direction.
i know how to but without the play button. go to this link http://web.roblox.com/Intro-GUI-Creator-item?id=142427411 then download the plugin.(you must be in rob lox studio) then enter the world you want it on and click the "create intro gui" and go to the script and enter in the needed formula! (it will tell you what you need to do in the script)