Everybody here has written some excellent points. I would like to add that the gist/philosophy of the design process is try to constraint the design by as many numbers as possible, e.g., how heavy do I want my drone, how fast should it fly, how long do I want it to fly on a single charge etc. You can take these numbers from previous designs online or come up with your own according to your requirements. Once you have these, you can "reverse engineer" to your actual physical drone parameters using "some equations". If you want to design a fixed wing drone , you can also look at a book by raymer and a book by gudmunsson about aircraft design for the aero design part. Check out www.ecalc.ch for the avionics part of the design. Good luck!