Fixed bar path, all the focus on driving up.
Push through the top foot, don't push off the bottom one. Let the working leg do the work.
Step-ups usually make you split your effort between driving up and not tipping over, and the Smith machine deletes the second half of that deal. With the bar fixed on its rails, the working leg's quad and glute press you onto the box with total focus, one side at a time. It is the rare unilateral leg move you can load heavy without balance becoming the limiting factor.
Balance stops being the bottleneck. With the bar on rails you can load heavier and take each set closer to real leg fatigue, where dumbbell versions often end early because of wobble or grip. The trade is less stabiliser work, which is fine when leg strength is the goal.
High enough that your thigh is around parallel to the floor when your foot is planted, roughly knee height for most people. Higher increases the range and difficulty, lower lets you load heavier, both are valid depending on the day.
Slow the touchdown so the bottom foot lands quiet, then drive from the top heel with the back foot as dead weight. If you cannot, the load or the box is too much, dropping either fixes the cheat instantly.
