To find it, look for a small, usually red, button or switch often near the fuse box or other electrical components under the dash. You might need a flashlight and possibly need to remove some trim panels to access it fully.
If you can't find it under the dash, consult your owner's manual. Failing that, a quick online search for "2001 Ford F350 inertia switch location" along with images might help you visually identify it.