How to do auto complete in vim?

Ctrl + N

(No plugin needed)

Vim auto complete

Type in a few letters, press Ctrl+n (next), or Ctrl+p (previous), a list of candidate words appears, which are all the words in the file, beginning with the a few letters you have typed in.

If the first candidate word is what you want, then you can continue to type in anything you want next.

Otherwise, you can continue to press Ctrl+n, or Ctrl+p, until the candidate word you want, then continue to type in anything you want next.

:help popupmenu-completion
:help popupmenu-keys