Watch the screencast that accompanies this TIL to see how to do this visually.
On macOS there are a set of keyboard shortcuts for taking screenshots. The one
that I use most often is Cmd-Shift-4. It turns the cursor into cross hairs so
that I can make a drag selection of part of the screen to be captured as a
screenshot.
I've recently transitioned to using CleanShot X for taking screenshots. It
offers some improvements over the built-in screenshot tool, like annotations
and sticking recently screenshots in the corner of the window. As part of this
transition, I wanted Cmd-Shift-4 to activate CleanShot X's Capture Area
feature instead of the built-in one.
To do this, first open up System Preferences > Keyboard > Shortcuts. Then go
to the Screenshots section and de-select any shortcuts that you want to
remap. For Cmd-Shift-4, that is "Save picture of selected area as a file".
Then I open Preferences > Shortcuts for CleanShot X and map Capture Area to
Cmd-Shift-4.
Hitting Cmd-Shift-4 will now activate CleanShot X's Capture Area instead of
the one for the built-in screenshot app.