I found airing my tires with nitrogen didn't need to be as difficult as changing a tire.  Basically,
you just need to empty the tire of all air ─ and then fill it up with nitrogen.  And, under my vehicle or trailer, I placed a bottle jack [like the one shown below], whereas you might use something different.

You don't need to "jack up" the vehicle. You need something that will support the frame behind the tire you're going to empty of air, as it comes down. The point is to not allow the rims [wheels] to come down on the tire, all the way to the concrete pavement. That might possibly damage the tire. But you DO want to remove all the air from the tire, before inflating with nitrogen ─ otherwise, there is no point to this exercise, right?

Airing tires on a vehicle

If you are airing a vehicle, first set the vehicle on a level surface, not a sloped surface. Make sure the vehicle is in "PARK" (or in "First" gear, if you have a manual transmission), and that you've applied the emergency brake. Some vehicles' owner manuals actually diagram, or describe, where the jack points are located on that vehicle. Check to see if your owner manual [or, at best, online] describes your vehicle's "jack points", for proper placement of the jack under the vehicle.

If your manual is incomprehensible, or lacks jack placement information, try to place the jack so that it touches either the vehicle's frame or the big bar that supports the front wheel suspension. Look under your vehicle for the point in the car frame behind the tire you're working on. Place the jack under this point (just to the right of the rear tire -or- just to the left of the front tire), so you can visually see the jack during the inflation process.

Jacks are used only to get a vehicle off the ground. They should never be used to hold a vehicle in place. Applying the emergency brake works on only the rear brakes. When you are working on rear tires, place one wheel chock in front of the rear tire you're NOT working on, and one in front of the front tire opposite to the placement of the rear chock, if your vehicle is level or on a slight forward down-angle. When you are working on the front tires, place wheel chocks in front of both rear tires. Note: Jack stands are normally used for added safety when you'll be working under the vehicle. [You should never need to work under the vehicle for the process of deflating and inflating your tires.]

Position the tire's valve stem to the lower right, or lower left, side of the tire (whichever is most comfortable for you to be able to apply pressure against it, when inflating). The valve stem should not be squarely on the bottom of the tire, where the pavement can squeeze it after deflation of your tire.

I raised my bottle jack's height, prior to placement, to about 1/2" under that frame point, leaving a small 1/2" gap. Then, using your tire-pressure gauge, slowly let the air out of your tire. As you do this, the frame of the vehicle will lower, coming down to rest on your jack. Once it does, and you see that it has seated firmly onto the jack, then remove all the air from the tire.

When all the air is out (you don't hear any more gas expelling from the tire), then apply your nitrogen inflator chuck to the tire. Check the tire's pressure intermittently, using your pressure gauge, to see how fast the tire is coming up to pressure. Lastly, keep an eye on the jack as you do this ─ when you're "back" to your tire's previous pressure, you should notice that the vehicle frame has again risen above the jack by about 1/2".

Finish the tire's inflation by using your tire pressure gauge's reading. Your tire may have been low on air when you began. Always fill the tire to the pressure indicated by the model of tire, or your car door's specs (put there by the manufacturer). The only time you might not air a tire to the amount of PSI stated by the original sticker on the drivers-side door is when a set of different-size tires has been mounted to the vehicle, other than those of its original manufacture.

Airing tires on other things

If you are inflating tires on a trailer, I suggest you first hook the trailer to a vehicle for stability. The vehicle will keep the trailer in place. If you cannot do this, then make sure your jack is placed in the most optimum position for stability under the frame of the trailer, and that you place chocks, bricks or wood blocks in front of, and behind, the opposite-side tires (than the side you are working on) to minimize movement or rolling. This, of course, is most critical the lighter the trailer is, as these move or roll the easiest.

If you are inflating tires on a motorcycle or scooter, make sure to put the cycle up on its stand (not kick-stand) before proceeding with inflation.

Alternatives

If you wish to fully jack up your vehicle or trailer, just remember that you only need to raise it no more than 1" off the ground. This allows you to "roll" the tire, positioning the valve stem where it is most comfortable for you. However, when the parking brake is "on", you will not be able to roll the rear tires, anyway.

As in dealing with cylinder gas tanks, have respect for heavy mobile items that run on tires! Be safe, don't get in a hurry, don't get distracted, and always think about what you're doing.