for units:
select unit(s)
while holding alt, right-click where you want the order
You can also hold shift if you want. If you hold shift, it will add the attack-move after whatever orders the unit already has. If you don't hold shift, the attack-move will become the unit's only order.
for factories:
same thing, select the factory/factories you want, hold alt, right-click
"factory attack move" (FAM) is a special kind of attack move. Engineers on regular attack-move (RAM) behave exactly like engineers on patrol. So they wander around, move up to reclaim, reclaim it (if your bars aren't full).
FAM engineers will stand still at their waypoint, and reclaim with greater range. This is great because engineers can't reclaim while they move. So you can take more reclaim faster and you can have engineers further back away from the enemy.
To get FAM, your factory can only have ONE attack-move order. It can have any number of move orders before the attack-move order. (zero, one, two, etc.) but only one attack-move order. If you put more than 1 attack-move order, engineers coming out of the factory won't have FAM, they will have RAM.
https://www.youtube.com/watch?v=q2WFQxymNMA&list=PLJclJGPtIxW1Znl4idf8EEdHMnTzDJ810&t=1h43m31s
On a map with mass that can be reclaimed, like stones, you need to have factories that send out engineers to grab the mass. You can use a patrol order, which you don’t need to worry about, or you can do an attack move order, which gives the engineers extra reclaim range. If you use attack-move, you then press shift so you can see the attack move order and drag the factory attack move order to another place. If you keep moving the order around, the engineers that have been produced will then keep moving and reclaiming, but this takes your APM. When you have reclaimed about half of the stuff that can be reclaimed, stop making more engineers for reclaiming. At that point, let your existing engies finish reclaiming the things.
If you watch replays from higher-rated players or even a lot of medium-rated players, you can watch from their POV. If you hold shift, you will see their orders. Depending on the map/situation, you will often see them move the factory attack-move orders around so engineers will reclaim most efficiently. The classic example is on Seton's, where people reclaim trees at the start, but you also see this commonly when there is a big reclaim field like after a big battle.
