There are various methods to put bearings on a wheel. But the simple and easy way is to get wheels out of skateboard trucks, place one bearing in the truck, and then put the wheel on it and press until the bearings get fit in wheels. Then put another bearing in trucks, flip the wheel, put it again in trucks, and press until the second bearing gets into it on another side of the wheel. Follow the same method for all four wheels, and after that, put the nuts and tighten the wheels.