First, you should start by assembling the frame with your newly soldered base.Try to pass each pair of wire of the ESC trough the hole under the each arm of the quadcopter.

Afterwards, you can mount the each motor on the tip of the arms. The holes for the screws of the motors don't fit perfectly with the holes on the frame so you will have to use only 2 screws to fix the motors. Also, direct the wires of the motors to the center of the drone and not outside to make sure you will have enough length to do the connections between the motors and the ESC.

Thereafter, place your Raspberry Pi support on the frame using bolts and nuts and fix the the Raspberry Pi on the support and the multiwii. Use the mounting pad for the multiwii to absorb the vibrations and place it exactly at the center of the quadcopter and facing the middle between two arms of the same color.

Place the receiver next to the multiwii with some velcro.

If you haven't bought a support, just fix your parts with velcro or some tiewrap. Just ensure that the multiwii is in the center of the drone.

Next, place an ESC on each arm with the 3 wires facing the motors. Connect the connectors between themselves and and fix the ESCs on the arms with some tiewrap.

Finally, place the battery between the two stages of the frame, attaching it to the top one with some straps.

You're done, everything is now on the frame.👍