One of my 3 children was fascinated by letters, numbers, reading and writing when he was very young. I showed him upper case letters, which are easy to form for little children, and he had lots of fun writing little messages and silly words. Just think how easy it is to draw an upper case E or A with a paint brush or marker. This is more attractive to little kids and easier to do than e or a.
I never told him how to spell things, but just had fun with him letting him make up his own spellings and play with putting sounds together. He liked drawing too, and the letters and writing were part of that. He would write little messages decorated with pictures and really got the idea of letters representing sounds and getting stuck together to make other sounds when he was very young. It was joyful and fun.
He did end up being very academic and doing extremely well in school, and I am sure his playful introduction to letters and numbers helped make that happen.
Before going to school he didn't have the idea that there was a right and wrong way to spell, but he did have good phonetic spelling. As soon as he was in school he learned to spell very easily.