Make sure the switch is rated to handle the motor current. At 110V, a 15A switch is good up to about 1hp. If your machine doesn't track perfect at start up, congratulations, it's just like all of mine. Granted that every thing is reasonably rigid and aligned and the belt is some tension, the belt will not wander due to the crowned rollers. However, initially the belt may not track in the center of the rollers.
Frustration is usually due to over compensation.
A little adjustment goes a long way here. You'll quickly learn. Now that it's completed, notice the designed features. The belt support was notched out to provide clearance. Basically, the belt is unsupported.