Get a backpack bigger than the free starter bag first. The FOB is a 3x2 item and will not fit otherwise.
Where to buy it is disputed. The demonstrated route is from a vehicle that can hold items: press B, go to Items, open the Building tab. One creator says the base vendor sells it; another says it does not. Use the vehicle menu. An ATV will not work because it cannot hold items.
Drop the FOB where you want to build. It creates a square build zone around itself. Place a structure silhouette, confirm it, then swing at the yellow X marks: about 10% progress per X against 1-2% per blank swing. Click once per swing instead of holding the button, and stand at maximum reach so the next X spawns close.
Supplies in your truck do nothing. Drive into the build radius, press G once per pallet, then get out and hold F on each dropped pallet to deposit it.