There's no exact way to fit all the washers and spacers. Even if there were an instruction, it wouldn't work because every bike is different. Here's how I fit a motor:
First, file the drop-outs first to get the motor into the drop-outs. I normally file the drop-outs about 2.5mm deeper to re-centralise the axle otherwise the tab washer doesn't engage with the slot and your brake pads might not line up with the friction area of the disc, though every bike is different, so I have to make a judgement on each one.
Next job is to get the disc to line up with the caliper, so I put the caliper in the middle of its sideways adjustment slots and put arrange any washers or spacers to position the disc so that it lines up with the gap between the pads.
Then I move to the other side and arrange any spacers and washers so that the cassette is just far enough from the drop-out to stop the chain touching the frame. That adjustment is vital to prevent the rim being off-centre. The clearance needs to be as small as possible.
Finally, check how central the rim is in line with the frame, then dish the wheel, if necessary, to make it central.
When arranging the washers and spacers, it helps to have a box of old 12mm washers and spacers of different sizes and thicknesses. The tab washers can go on either the inside or outside of the drop-out to suit, and you often need a non-standard 20mm O/D washer on the cassette side to go into the cassette without rubbing, though the Bafang type anti-rotation washers can be used for that when placed on the inside of the drop-out as long as it let's the chain close to the frame. It depends where the axle nut is inside the cassette. Some are nearly flush, others are deeper inside.
