Print one hub piece and four each of the other pieces. I suggest you print a few spare legs as they will break upon a rough landing. This is by design, to absorb a bit of the impact.

Cut four lengths of flyscreen frame bar. I used 200mm lengths, and this is slightly too small for 9 inch props. I apologise for mixing units there, but propeller dimensions are measured in inches, :(.

Fit the motor mounts onto the ends of the rods, making sure to get the polarity right. Make sure you really hammer them on. Measure the length of your arm to make sure it's fully seated into the mount. If you're feeling uncertain about the fit, put a M3x20 screw into the screwhole in the end to lock it in. If you're still feeling unsure, hammer in one of the hub_junction_channel() pieces too.

Slip the sleeves over the other ends of the arm, sliding them down the arm about 20mm. They may require some persuasion. Attach the four arms to the hub, then slide the sleeves back inwards towards the hub. Insert the hub_junction_channel pieces into the channel and hammer them inwards to lock it all together.

Clip the leg pieces onto the arms, near the end.

Done! Your quadcopter frame is complete. Attach motors, electronics and props!

Please note that quadcopters are inherently dangerous pieces of equipment. Please be thorough in your testing with props off to get your directions of spin and calibration sorted out. Having your legs cut to ribbons by a rogue quadcopter is nobody's idea of fun.