In TK- K the transition begins with scribbles, then moves on to squiggles, symbols that look like letters, letters that are recognizable, and finally phonetically spelled words and phrases.

Stages of writing development:
Preliterate stage: Scribbling is good (0 to 2 years)
Emergent stage: Letters appear in writing (2 to 4 years)
Transitional stage: Letters start to become words (4 to 7 years)
Fluent stage: Spelling starts to have meaning (5 to 6 years)

If you need tips of strategies to move your students from one stage to the next I would be glad to send some information on that as well.

I see this question has already been answered but I just wanted to offer up some comments that hopefully others will find helpful:
-I highly recommend Handwriting Without Tears as a resource for teaching how to print letters of the alphabet.
-Letters are best taught out of order when it comes to learning how to print them. Letter “A” for example should not be one of the first letters taught as diagonal lines are a more advanced fine motor skill than say the parallel lines in letter “H”. What I appreciate about Handwriting Without Tears is that the materials and lessons are organized with this in mind.
-When teaching phonemes (the sounds letters make), the ABC order is likewise not the best approach. “S”, “A”, “T”, “P”, “I”, and “N” should ideally be taught first as they are most common in CVC words and early reader texts. Once a student knows those letters’ phonemes they can already start reading! (words such as “sat”, “pat”, “nip”, etc.