How to do:
1. Install the Toastr app here.
2. Run the app.
3. Add a message to float as a reminder when you unlock your phone.
4. Choose whether it should float for 1.5 seconds or 3 seconds.
5. There's a Pro version, which costs 99 cents; the Pro version lets you change position, color, etc.
6. Click "Start toastr" and you're good to go.
7. Now when you unlock your device, your message will appear as a reminder. To change the message or stop the pop-ups, just restart Toastr and make your changes.