Tighten the end plug until the spring is compressed fully, then back off just enough to insert the cotter pin through the slot. You'll need a "drag link socket" that resembles a big screwdriver blade with a square drive hole for a ratchet. We would sometimes drill a second cotter pin hole 90 degrees from the first one so the plug wouldn't have to be backed off as far, like having two cotter pin holes at right angles for the castle nut in some spindles.

If you're talking about the nut that attaches the Pitman arm to the steering gear, always keep tightening the nut until a cotter pin slot lines up in the castle nut- - - -never back off. That big nut takes a very high torque- - - -150-200 ft/lbs or so.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!