It depends on how you want your intro and outro look like. The most simple variant is a short video clip you prepare. One for the intro and one for the outro.
Create one scene for the intro, one (or more) scene for your regular stream, and one for the outro.
In the intro and outro scenes, add a media source for the corresponding clip. Activate "Restart playback when source becomes active". This way the clip will play from the start when you switch to that scene.
Create another scene, a "Starting soon" scene. In this scene put an image source with a static image, showing a message like "Welcome! Stream is starting soon." in a more or less fancy way.
With that, you can run your stream like this: While you get ready yourself, start OBS and change to the "starting soon" scene. Then click the "start streaming" button. Now your online status is announced to your prospective viewers and they chime in. They will see the "Welcome! Stream is starting soon" and will wait until you start to perform. You get yourself ready, prepare your desktop and do whatever you need to do, and after 5 to 10 minutes you switch to the intro scene. The intro will play back automatically. After it finished playback, you switch to your regular stream scenes and perform whatever you intend to perform. After you have finished, you switch to the outro scene and the outro will play back automatically. After that, you click "stop streaming".
