Make plans. After that, just go with it!
It started with Norah asking how to make a "G" the other day. This turned into an impromptu handwriting lesson on her white board. After I reviewed how to form that "G" she asked for to begin with, she said, "Let's do all the letters!" So, I printed the letters in no official order, capital and lower case, and she used a different color marker to practice writing them below mine. She got handwriting practice I hadn't even planned, all the while, thinking she was just playing a game with Mommy.
I don't think this kind of student-lead learning can take place without parents providing some fuel for it in the first place. Right now, I am providing Norah with an age appropriate, non-mastery K4 curriculum. It covers basic skills like how to form the letters, how to hold a pencil, etc. but it doesn't expect perfection from little hands that can't give it and doesn't even provide a single sheet of lined paper for drill at this point.
These simple lessons have served to peek Norah's curiosity and fuel her interest in handwriting. Without the lessons, she may not have asked how to make a "G" or even had the confidence to give it a try at this time in her life when it becoming pretty reasonable for her to do so.
So, I say, "Make your plans" and make sure they are reasonable, considering the age and skill level of your child. But, after you've done that, feel free to "Just go with it" and let your kids follow the interests that information is bound to spark within them.