You'll likely need to feel around under the dash with your hand to locate a small, usually square, black box with a button on it. Some are easily visible, others may be tucked away a bit.
If you can't find it easily, consulting a 1989 Ford F250 repair manual or searching online for images of the inertia switch location in that model year would be helpful. You can search specifically for "1989 Ford F250 inertia switch location" on Google Images.