First insert the o-rings.

Then add the washers.

Eventually, close slightly with the nuts.

Before screwing firmly the nuts, shake a little the assembly or allow a few kicks with a soft mallet on the back of the pump, to adjust the mechanical parts to each other.

If you can, test that the motor rotates freely by connecting it to the power supply right now. This is a useful step to do right now, because if the brushes are not correctly assembled, you'll have to disassembly the pump again e start from step 4.

Firmly fix the nuts, but be prepared to untighten them as needed if the pump draws too much current.