I'm not really sure what to tell you. If you tap X when giving an order, it cancels the current order. If you hold X, it queues the order instead.
For example, say you want your Warthog or Ghost to scout the map. Since you haven't given any orders yet, you can tap or hold for the first command - move to the front of the enemy base, for example. Then, move the cursor to one of the corners; press and hold X. Now move to another corner, and repeat - press and hold X. If you tap X when giving an order instead, it cancels all queued waypoints and immediately executes the new order.
It also works for gathering supply crates. You can scout out the map, then once you can see the locations of the supply crates, you can queue orders to gather them all. I don't know if you can queue attack targets this way or not, never tried it.
I don't use long queues like that for assault units, but it seems to work differently for them than for recon units. The one time my assault units ran into an enemy patrol, it seemed like they forgot their waypoints and just stopped to fight. I'd have to play with it more to be sure.
