Place one bearing into the wheel, press the wheel down evenly and repeat for the second bearing. Keep pressure centred on the bearing, not on the shield or seal.
Use the hardware correctly
Spacers sit between the bearings and speed rings sit between the bearing and the truck. Keep the order consistent and do not force misaligned parts together.
Final check
Tighten the axle nut until the wheel has no dangerous side-to-side movement while still spinning freely. If the wheel binds, back off slightly and check alignment.