Looking for advice on how to move stepper motor on TTL signal reception?
There are plenty of simple stepper controllers out there that take a TTL step signal and advance the motor a single step and a second signal for direction. They often allow a user definable level of micro-stepping.

You can either buy the chips (normally surface mount with a big pad underneath the part so they get better heatsinking to the pcb and so tricky to hand solder) or you can get modules with the part on that you can then connect to.
e.g. https://github.com/watterott/SilentStepStick

You then need to add the software control to pulse the step signal at the correct frequency to give the rate of turn you require. Constant velocities are trivial, getting the timing correct for smooth accelerations gets a little more complex but isn't too bad.

Knowing your current heading is simply a case of counting the number of steps assuming you haven't slipped due to trying to accelerate too fast.