Designing in 123D is super easy and it is by far mt favourite program for quick models and the fact that its freeware is just perfect! I began designing regularly recently and recording my designs as well as speeding them up to make them easier to watch take a look!
Step 1: Get Your Measurements!
The biggest issues I always have is the measurements so get all of these before hand, in this case the motor mounts and the flight controller mounts. Everything else can be made aesthetically around these measurements. to "look right" i wasn't too worried about making a perfect drone so I simply made what looked good to me.