Summarize this article:

The most popular and current is Lock Screen OS 9 from AppPro OS Studio. Open the app’s settings, then tap the checkbox next to “Enable Screen”. You can go back to your original lockscreen by unchecking this box later.  Be sure to Disable System Lock before activating the app. Different apps have slightly different steps involved with activation. Be sure to follow each apps instructions, usually listed in the Google Play store page for that app. This allows you to set an iOS-style pin for your phone’s security. From there, you can select from a variety of wallpapers for your iOS 9-themed lock screen.

Summary:
Launch the Play Store. Download and install a Lock Screen app. Activate Lock Screen OS 9. Tap the “Pin Lock” checkbox. Tap “Set Wallpaper”.