This will log into your Microsoft account! If you do not feel comfortable with this, launch Minecraft and go to Settings -> Skin Customization -> OptiFine Cape -> Open Cape Editor.
Make sure you look at this domain and see if it's the right one. You can do this by going to OptiFine's official website and seeing if it links here.
Click the random banner to see your history!
Import PlanetMinecraft Design
Use the "Shareable URL". If it's an existing design, click "Remix Banner" first.
**NOTE: This does not use the new patterns or colours. It's limited from OptiFine's end.**