"How to train for western pleasure" is a loaded question. There are so many specifics that cannot be written in a forum post. I would suggest to get the basics down with walk, trot, canter, back, pivots, sidepass, lateral flexion, vertical flexion, & leg pressure, then when you can find a trainer to help you, you'll be ready to move on to the WP training.
I would not use a tie down to get her to keep her head down at this point. It sounds like she's just learning things & she will eventually learn to keep her head where you want it when lateral & vertical flexion are taught to her & you ride her correctly giving her a release/reward at the appropriate times.
Perfect Jin N Scotch - 2004 APHA Palomino Overo Gelding - Western Pleasure and Showmanship
Hanks Rainy Sky - 1998 Black and White Tobiano Paint Gelding - relaxing rides and blazing trails