The "montessori read and write" book is a great reference. The language training is quite in depth.
I'm going to try to do this succintly. It's hard to single out any one lesson, since they are all connected. For Montessori, there are 3 branches of learning language:
1. Spoken language - includes oral stories, songs, fingerplays, conversations, question games, word play and vocabulary lessons. We prepare the for writing and reading exercises with spoken language activities. The child builds upon the spoken language lessons when moving on to the other two areas which are:
Montessori breaks every skill down into the component parts, introduces one skill at a time and builds in complexity. For writing and for reading there are indirect preparations in the practical life and sensorial materials. The Metal Insets are a direct preparation for writing.
The Sound Games are first in the progression. First the child learns that words are made up of sounds, and learns to "dissect" a word into the component sounds. Next, you unite the sound and the symbol with the sandpaper letters (given in 3 period lesson w/ 3 letters). Once the child knows about a 3rd of the letters, you can move on to the moveable alphabet. This progresses from words, to phrases, to sentences to stories and there are 3 different sizes. The child is not necessarily ready to write with paper and pencil. Nor is he ready to read. Usually about 6 months after work with sandpaper letters, moveable alphabet, sand tray and chalkboards - the child will begin to read back what he has written. This is the beginning of the reading exercises which progress from simple phonetic activities to phonogram and puzzle word work to functions of words and sentence analysis. It is commonly referred to as a "Total Reading" program because through work with these materials the child becomes fluent in these different aspects of reading (sounds, spelling, word placement and function) in the 3rd year.
Writing continues with the child using the phonogram sandpaper letters and the chalkboards, this teaches connections. The child is never asked to write on paper until he himself initiates it. Once, he does, we start with one word books, labels and short phrases on large strips of paper. The paper size begins to decrease and lines are added as the child begins to show more control.
Well I guess that wasn't too short. To answer your question, I would move to the moveable alphabet next. It is a parallel activity to continued work with the sandpaper letters and also the chalkboards.
My sweetie and I have a lovely little lady 07/02 and 3 cats