Result of monad inside monad transformer

Question

This is my first acquaintance with Monad Transformers, so the answer might be obvious.

Let's say I am inside a do block of type StateT MyMonad MyType, I want to make another function of the same type both modify the state and return a value of type MyMonad MyType. How can I achieve that? I think the examples here show it in guessSession, yet I can't seem to understand how to apply it!