You will need:

1 Add velcro to base plate

Thread the three velcro straps through the slots in the base plate. These straps will hold the battery to the bottom of your copter. The perfectly flat side of the plate (without the protruding nuts) should face down. Your kit includes two velcro strips to attach to the battery and the bottom of the copter. To install, attach the strip with loops (fuzzy) to the bottom of the plate between the straps, and attach the strip with hooks (smooth) to your battery. Click to visit the APM:Copter Wiki and lean more about which batteries to use with your copter.

bottom side of base plate with velcro

2 Attach arms to base plate

Now we’ll attach the four arms with assembled motors to the base plate. Using the innermost hole in the arm and the hole in the base plate marked below, insert a 30 mm screw from the bottom, up through the plate and the arm. Attach the two black arms and two blue arms as shown in relation to the shape of the plate. (Attach the black arms to the more elongated end of the plate.) It may be helpful to secure each arm in place temporarily with a metal nut.

3 Label motors

Use tape (or other method) to label the motors by the numbers shown below. (The number above the line corresponds to the top motor; the number below the line refers to the bottom motor.)

4 Connect ESC and motor cables

Connect the motor cables to the corresponding ESC cables according to the labelled numbers (connect motor 1 to ESC 1, motor 2 to ESC 2, etc). The order of the three cables determines which direction the motors will spin and is critical for the copter to fly correctly. The diagram below shows which motors need to be wired in configuration A and configuration B according to which arm; connect both motors and ESCs on arms labeled A in configuration A and on arms labeled B in configuration B.

The images below show the correct wiring to produce each direction of rotation. The motor cables are differentiated by color. To differentiate the ESC cables, orient the ESC with the label facing up, and use
the relation of the cables to the label.
configuration A wiring

configuration B wiring

5 Assemble electronics onto base plate

The base plate holds the power system: power distribution board (PDB), power module, and four electronic speed controllers (ESCs). Place the assembled power system onto the base plate. Attach the four ESCs to the base plate using zip ties. Orient the ESCs with the labels down into the holes in the plate. Alternatively, you can mount the ESCs to the front of the copter, perpendicular to the plates, by securing the zip ties to the small slots along the front edge of the plates. In this case, orient the ESCs with the labels facing out. Ensure that the power module XT60 connector protrudes from the back end of the copter (behind one of the black arms) where it can easily connect to the battery.

6 Assemble top plate

Remove any nuts that you may have used to secure the arms temporarily to the base plate, and add the top plate directly on top of the arms. Secure the four existing screws with metal nuts, and add a nylon nut on top of each metal nut. Secure the arms to the plates through the holes in the end of the arms and the holes in the plates shown below. Use 30 mm bolts and thumb nuts to attach the black arms, and use 25 mm bolts and metal nuts to attach the blue arms. Finally, attach the base and top plates together with 25 mm bolts, 19 mm hollow spacers, and metal nuts where marked below.
top view
bottom view

7 Thread cables through top plate

Locate the following three cables, and thread them up through the central space in the top plate: PDB two-wire cable (red and black), PDB four-wire cable (multicolor), and power module six-wire cable (red and black). These cables will connect to Pixhawk.