Chances are you won't easily get roadside assistance with a trailer, so pull off the road as far as you can, put on your safety vest and leave the horses in the trailer munching on their hay.
Be prepared to get dirty.
Things to check before you leave home: does your truck wrench work on your trailer tires? If it doesn’t there’s a nifty universal telescoping lug wrench from Canadian Tire (part number 009-1517-8) that will do the trick.
Loosen the nuts before you jack up the trailer or you’ll be literally spinning your wheel. Use some leverage from the wrench, using your foot if you have to get some more power behind you.
Place the jack under the trailer, as close to the flat tire as possible and keep it vertical. It may help to place a piece of wood under the jack to stop it from tipping in the dirt. There is a handy wheel chock ramp called a Trailer Aid ($60), available at tack shops, that can be used instead of a jack. That's what I'll be using.
Remove the nuts and place them on the fender, not in the dirt! Keep in mind how everything came apart and that’s how it will go back together. If you get confused, look at the other tires as an example.
Take off the flat and put on the spare (you did check before you left home to be sure you have a spare, with air?) with the valve stem facing out. Put the nuts on and tighten them in a ‘star’ formation so that they tighten evenly, using as much pressure to tighten them as it took to loosen them.
Lower the trailer and you’re good to go! Check the nuts after 100 km to be sure they’re still tight.
You should be done by the time the hay net is empty.