I originally built with the extruder plate upside down (easily done with the dualstruder) so here's how to avoid that in order to have your levers for opening the filament roller on the outside (most reachable) sides, rather than in the middle where it may be crammed between wires. Also it allows you to align your nozzles on the Y axis so that you only have to configure an X distance between the two.
If you look at the plate so that it resembles an M, you'll be standing in front of your printer (LCD side forward, power side backward) when it is fitted. If you look at the two cutouts, you will see that one has the screw holes closer to the outside of the plate - further away from the bottom of the U shape. That is the left side as you look at your printer from the front: the side closest to the X axis motor and limit switch.
The extruder can be fully assembled apart from the heater block. It is easier to insert that once you have the extruder in place (but not screwed in)
You will place your first extruder so that the fan faces the power supply and the rear of the stepper motor faces you at the front of the machine. The easiest way to insert it is to turn clockwise 90 degrees, place down into the U until the PEEK block is aligned for height with the plate and then turn 90 degrees anticlockwise before screwing in place. Just as a confirmation, the aluminium "cold end" will mount with the side closest to the stepper pretty much aligned with the edge of the plate on the left (outer) side. The second extruder faces the opposite direction.
If you cannot insert your first extruder (left side of M) into the U and reach the screws, double check your plate is the right side up.