Find a couple of wood blocks stack them so they are high enough for the rear tires to clear the ground, and place them in front of the front wheels on your trailer. pull the trailer up on the blocks with your tow vehicle, and remove the rear wheels. It would be a very good idea to loossen the lugs on the wheels before doing this.
Get those tires mounted and balanced, and put them on your trailer. Then, do the same for the front wheels. Piece of cake.
I make it sound easier than it is, but not much.