Sure, here are the steps on how to customize your Spotify playlist cover:

1. Open your Spotify app on your desktop. Unfortunately, you can't change playlist covers through the mobile app.

2. Go to the playlist you want to customize.

3. Hover your mouse over the playlist cover and click on the pencil icon that appears in the top right corner of the current cover image.

4. A window will pop up, allowing you to edit the details of your playlist. You can change the name, description, and most importantly, the cover image.

5. To change the cover image, click on the current cover image in the editing window.

6. Select the image file you want to use from your computer. The image should be a .jpeg or .png file and should ideally be at least 300x300 pixels for the best resolution.

7. Once you've chosen your image, click on 'Save' to finalize your changes.

Remember, Spotify's guidelines state that the image you use must not violate any copyright, trademark, or personal image rights. So make sure the image you use is either your own or one you have the rights to use.