Thejoyfulpuddlejumper is on the money here- the term 'child-led' is a bit of a misleading one, it doesn't mean that there are no boundaries or discipline. Quite the opposite in fact. It is acknowledging everything a child needs to learn (i.e academic, family, culture, emotional intelligence etc...) but asking you to observe & listen to your child to find the most appropriate time to teach it. Looking for natural teachable moments where the child would respond best to the information.
For example, if your 3 year old is very interested in cars, but hates the thought of 'writing' anytime you have dragged him off to do it. Sitting him down, forcing him, being strict, growling him- will not help him, it will not teach him, it will only increase his hate of writing and not to great things for your relationship either.
A child led example would be to join him where his interests are. Play alongside him while he is playing cars. Maybe he mentions "My car is off to the zoo"
You might reply "hmmm how do we get there?"
He might say "you go up here and around this corner"
You suggest "I would like to come- maybe we can draw a map so we don't get lost- I don't know the way- can you show me?"
Here you have your in to mark-making. It is following his interests. He now will want to show you with a pen & paper "up here and round the corner"
The ability to write, comes from the skill of knowing how to both make intentional marks on paper, then the concept that those marks can be understood by others. Eventually children learn about the concepts of words & letters. That your A should look pretty similar to my A for it to be an A.
To get to that point (& enjoy the process enough to want to keep practising & learning) experimentation, not worrying to much about whether you make mistakes, and having the scaffolding support of someone who knows a bit more than you (but is not taking over) is AMAZING.
In this context- only your child knows the map. There is no right or wrong. Whatever he draws is correct. This type of play is a great foundation. As his confidence grows, so will his skill. By joining his play little & often, you get regular opportunities to introduce these concepts. Gently, but regularly. Not so it takes over the whole play session, but enough to plant a seed. It also does amazing things for your bond. He feels listened to, respected, like his ideas are good. It feeds the imagination & creativity. It promotes problem solving & eventually critical thinking. Something being strict & formal does not.