I want to put a big message on my home and lock screen. Something like "don't screw up your diet". You get the idea. How can I do that?
One way is to just make an image of your wallpaper with the words printed on it and then select the new image as your wallpaper. Like this:
In the lock screen you can put a message in Settings->LockScreen and Security->LockScreen Message. This way is in my Xperia M5 Android 6.0 but is similar in other phones or android versions
Go to settings> lock screen and then there will be a option to add text on the lock screen!