I actually do think that writing documentation is quote fun.

It's very similar to writing software. You have to try to keep it KISS, but at the same time be complete and without different parts doing/saying things that are mutually exclusive.

A good hobbyist can wing it a lot, because he/she has enough knowledge to make working decisions on-the-fly based on common sense and experience from past projects.

A beginner should spend quite a lot more time on putting down goals on a paper. Then these goals should be given weights - if they are required or just bonus features. This is required since the goals represents costs in money and time. And they also represents risks - the original design may fail even if time was spent in the planning. Finally, the goals should be expanded into rules: What are the requirements to be able to fulfill a specific target goal. Basically a form of check-list before starting to build the hardware, or buy the components/reference board.

By performing the above, a hobbyist will be better prepared for getting something to work. It will be a bit more work up-front, but collecting all facts will amount to a great boost in knowledge, and it will be more fun to later "connect the pieces" since the hobbyist will have a better understanding how everything will interact, and also be more confident that the result may work.

Example (obviously incomplete, but at least an idea):

Target goal: Should work in car during winter or summer.
- requires components specified for cold -> select automotive or mil-specified components.
- requires components specified for heat -> select automotive or mil-specified components.
- requires support for 10 to 15V supply voltage.
- requires (if power-hungry) automatic cut-off if battery voltage below safe levels, to make sure the car can be started.
- requires a box that can ventilate to avoid condense water.
- might loose power when ignition is turned of - or when engine is started - unless direct battery power is available.

Target goal: Should be mounted under car seat:
- requires holes or similar to fixate under seat.
- requires a maximum physical size.
- requires a minimum length for all connected cables.
- might require protection from interference from other equipment in car
- limits allowed noise level, to not interfere with car stereo.
- requires specific sound level for any beeper or similar.

Target goal: Should always manage to start after a reset.
- requires a working operation of the reset pin. Use reset chip instead of capacitor.
- requires the reset chip to have correct polarity for the reset signal
- requires stable voltage to the reset chip.
- might require watchdog kicks to the reset chip, if it has watchdog function.

Target goal: Should not break if supply polarity reversed.
- requires a diode to block incorrect polarity.

Target goal: Should be able to run software xxx.
- requires a processor known to run xxx - or knowledge how to rewrite software.
- requires a minumum flash size of xxx.
- requires a minumum xram size of xxx.
- ...

Target goal: Should be able to drive two relays.
- requires two processor pins.
- requires requirements for isolation voltage.
- requires max make and break currents to match the intended load.
- requires max number of make and break operations / second and during life.
- requires check if load is very capacitive or inductive.
- requires specific relay model decided.
- requires supply voltage enough to drive relay coil.
- requires free-wheel diodes, or relay with built-in diodes or MOS-FET transistor capable to sinking the energy itself.
- requires that number of deactivations of relay/second is lower than the free-wheel diode or MOS-FET transistor can support.
- requires consideration if power to relays lost when ignition turned off (and uC possibly still has power).

Target goal: Should have x inputs able to detect 12V existence.
- requires voltage divider.
- may require protection from polarity reversal.

Target goal: Should have y inputs able to detect 0V/5V existence.
- requires processor that is powered by 5V.
- OR: that is 5V-tolerant
- OR: a level converter
- OR: a voltage divider. Requires further check of allowed load impedance on input.

Target goal: Accidental 12V on 5V input should not break anything.
- requires that the input is read using ADC to allow a voltage divider always, stepping 12V to max 5V and 5V to max 2.1V
- OR: requires a series resistor and a 5.xV clamp diode.

Target goal: Keep track of time without supply power
- requires processor with RTC, or external RTC
- requires a battery OR supercap
- requires separate crystal unless uC is allowed to run power-consuming normal oscillator.
- separate crystal specification? What precision? Load capacitance? Temperature range?
- supercap also requires charging resistor
- supercap also requires diode to not discharge into Vcc
- if external RTC, reserve correct type of processor pins for communication.
- if internal RTC, reserve correct type of processor pins for separate RTC crystal.

Target goal: Measure vehicle voltage
- requires internal or external ADC
- requires specification of number of samples/second.
- requires that ADC supports required read speed
- requires that external ADC has protocol bandwidth for required read speed.
- requires that processor have dedicated hardware or enough capacity to bit-bang if external ADC used.
- requires specification of needed precision.
- requires specification of needed resolution.
- may require protection from reversed voltage.
- requires specification if measured voltage is the same as the supply voltage, or separate wire.
- requires experiment: must voltage be filtered? Analog filter? Digital filter? Digital filter increases needed sampling rate.

Target goal: All functions using one processor.
- allocate all specific pins
- verify in datasheet that specific pins not colliding in mode configuration
- allocate generic pins
- verify that there was enough pins
- verify that each and every pin really is generic and not open-collector or similar
- verify that any pin connected to 5V (for a sub-5V chip) really is 5V-tolerant.
- verify that any analog pin doesn't clash with a digital function.
- all pins that must be connected? Direct to ground? Direct to Vcc? Pull-down to ground? Pull-up to Vcc? Voltage reference? Specific need for inductors or capacitors? ...