The most basic preloader is a scene that is quite simple but still has some Flash ActionScript behind it to detect when the entire movie has been loaded into memory.

1. Load the movie that needs a preloader scene.
2. Click on Insert and select Scene.
3. Open the Scene panels window and select the new scene.
4. Rename the scene to something meaningful like preload and move it to the top of the list of scenes.  This tells Flash you want the preload scene to run first.
5. Open the last scene in your movie and note the last frame number.
6. Return back to the preload scene and insert two blank keyframes.
7. Double click on the first frame to go into the Frame actions panel.  Click on basic actions and double click the "If Frame is Loaded" option.  The parameters for this should be entered as follows:  the scene name parameter is the name of the last scene and the frame number is the last number from that scene.
8. Click the plus button to add a GoTo action inside the If statement.  Type in the name of the first scene of the movie and enter 1 as the frame parameter.
9. Double click the second frame and double click the GoTo basic action property - make sure the Go to and play option is checked at the bottom of the window.  The other parameters can be left as they are.
10. This completes the ActionScript portion of the preloader.  Now insert a new layer in the preload scene.
11. Select the first frame and click on Insert | New Symbol... , check the button marked movie clip.
12. Now you are in the library editing mode and you can provide whatever tweens - shape, motion - you want for the visual portion.
13. Return to the main editing window when you are done.
14. Open up the library and drag onto the stage for the first keyframe the new movie clip symbol.
15. That completes the basic preloader.  There are dozens of variations of this ranging in complexity from a simple message to a scene that has buttons.