The main way to use delta is to configure it as the pager for git: see Configuration.

Delta can also be used as a shorthand for diffing two files, even if they are not in a git repo: the following two commands do the same thing:

delta /somewhere/a.txt /somewhere/else/b.txt

git diff /somewhere/a.txt /somewhere/else/b.txt

You can also use process substitution shell syntax with delta, e.g.

delta <(sort file1) <(sort file2)

In addition to git output, delta handles standard unified diff format, e.g. diff -u a.txt b.txt | delta.

For Mercurial, you can add delta, with its command line options, to the [pager] section of .hgrc.