How do you get subtitles for mobile pocket edition?

This is not possible anymore. The offer to have gotten a free code for buying Java Edition is now expired.

In the create new world menu, just scroll down until you find the "world type" area. Then click on where it says "infinite" and select "flat" from the drop down...

Use an editing software to do that, such as Windows Movie Maker or iMovie. Import audio and text to add voices and subtitles.

Sadly, no. You can only do this in Java edition Minecraft, upon installing the Aether Mod.

Unfortunately, no. As of the time this answer is written (Dec 1, 2020), the Bedrock Edition (formerly known as Pocket Edition) of Minecraft still doesn't allow for custom Superflat worlds....

You apply the book to an item that you'd like to have that particular feature. For example, if you got a book with sharpness, you can put it in an...