First you need a idea what your os should do or for which people it should be made.
Then you have to choose a distro on wich your distro should base on(I would recommend arch).
Another question is if your distro should be a rolling distro or not.
You also have to choose a DE, a display-manager, a x-server etc.
You may also write special software for your distro, to make it a distro and not just a flavor.
Your distro also needs a need.
You can also make your distro more beautiful using themes or special configurations of for example your grub.