Let's say you just got a great deal on a vehicle because it's got flat tires.
You could have gotten an even better deal on a car with no wheels at all, chickens living in it, and a tree growing through the roof, but that's for later.
If at all possible inflate the tires before attempting to drag the car anywhere.
You don't want to dislodge the tire from the rim.
Little cigarette-lighter-plug powered compressors work pretty well for this, or this homemade compressed air can
If you already dragged the tire off the rim, or the tires were already that way, you won't be able to inflate them. Here's how to get the tire seated back onto the rim.
First jack up the car so there's no force on the tire.
Then wrap a rope around the tire as shown, put a stick through the rope, and twist the stick until the tread of the tire is squished down all around the tire. That will force the edges of the tire outward onto the rim. If the tire still is too loose for the regular air chuck to fill it, you can use an airgun to blow air under the bead of the tire as shown here. This tire is now inflated.
At my tire shop they use a ratcheting cargo strap around the tire for this, and in tough cases they use really large quantities of goopy greaselike soap around the rim. If you ask for a handful of that stuff they'll be really happy to give it to you.