With the keeper off (the one that locks the rims together), a tire tool or large screwdriver can be used to pry one side of split rim up and on top of the other side. This allows the rim to undersize and lets you do the tube and tire.
Some new tires seem to be a little undersize and then a rim spreader is helpful to push the tire back out where the two sides of the rim butt up at the split. I own a rim spreader, but I rarely use it.