I recommend buying and absorbing Countdown to Broke (Amazon
.com: The Modern Horseman's Countdown to..."> Amazon.com: The Modern Horseman's Countdown to...
It covers 33 steps in training to get to a reasonable broke horse, has very clear instructions and photos, and also has tips for deciding when your horse is ready for the next step.
Mia is 10 and was once used for endurance racing. I've had her for 2 years. After a lot of thought, I've started her back at the beginning because there were too many holes in her training. I'll continue to ride her, but we're also doing daily sessions to go step by step. The author figures with regular work, most horses can get there in about a year. Mia may go faster since she has already had a lot of riding, or maybe not - the holes may take longer to fill since she needs to unlearn some bad habits.
In any case, it is the best of about a dozen books I have on training a young horse (or a 10 year old with holes!)
He covers sacking out, but there is a lot to do PRIOR to sacking out. Skip those, and you'll end up with a horse like Mia - trained on the surface, but not at the core.