ExoPlayer is a powerful, flexible, and customizable media player library for Android applications. It's an excellent choice for creating playlists in your app. Here's a simple guide on how to do it:

1. **Add ExoPlayer Dependency**: First, you need to add ExoPlayer as a dependency in your build.gradle file:

```gradle
implementation 'com.google.android.exoplayer:exoplayer:2.X.X'
```

Replace '2.X.X' with the latest version of ExoPlayer.

2. **Initialize ExoPlayer**: In your activity or fragment, initialize ExoPlayer:

```java
SimpleExoPlayer player = new SimpleExoPlayer.Builder(context).build();
```

3. **Prepare a Playlist**: To prepare a playlist, you need to create multiple `MediaSource` instances, one for each item in the playlist. Each `MediaSource` can be created using a `MediaSourceFactory`.

```java
MediaSourceFactory mediaSourceFactory = new DefaultMediaSourceFactory(context);
MediaSource mediaSource1 = mediaSourceFactory.createMediaSource(MediaItem.fromUri(uri1));
MediaSource mediaSource2 = mediaSourceFactory.createMediaSource(MediaItem.fromUri(uri2));
// ... add as many MediaSource instances as you have items in the playlist
```

4. **Concatenate MediaSources**: Use `ConcatenatingMediaSource` to concatenate your `MediaSource` instances:

```java
ConcatenatingMediaSource concatenatedSource =
new ConcatenatingMediaSource(mediaSource1, mediaSource2);
```

5. **Prepare the Player**: Pass the `ConcatenatingMediaSource` to the player's `prepare` method:

```java
player.setMediaSource(concatenatedSource);
player.prepare();
```

6. **Start Playback**: To start playback, call the `playWhenReady` method:

```java
player.setPlayWhenReady(true);
```

Remember to release the player when it's no longer needed, typically in your activity or fragment's `onStop` or `onDestroy` method:

```java
player.release();
```

That's it! You've created a simple playlist with ExoPlayer. Note that ExoPlayer supports many more advanced features, like playlist reshuffling, repeating, and gapless playback.