It's not really that hard... I will take each part in turn.. Power supply... Micro essentials.. ADC inputs.. and use known working examples.... You usually work to a preset path... Normally you are asked to fill a space, you design to fit that space... After several systems, you find designing a new one, really isn't !! All your new designs are a variation of a theme..How are designs usually carried out? Do you design to certain specifications or do you design what comes to mind and figure out the specifications once you have all the components?