Place FOB in a place that can be easily defended, granted you have the man power and they are watching 360 degrees of your FOB. The build range on FOB is 50m, which is pretty far this is the same distance as Rally point/FOB if the enemy is near. So the further out you put the emplacements the better, given your teammates defend those area's appose to just camping the 10m around the FOB.
If your team is accurately placed at these fortifications, this will not allow the enemy to place rally points to close to your FOB, giving your TEAM more time to react to enemy assault.
Establishing a decent size perimeter is key to not being overwhelmed by one enemy who can pretty much pin down a whole team if team is stuck in a small area.
With this said i will give a few examples of places to throw down sandbags so your FOB does not get overhelmed easily.
Blue=FOB Green= Good sandbags Red=Bad sandbags
(Red does not directly mean dont put sand bags there, it just means you want to focus on establishing the perimiter so TEAM can get in and out of FOB with out taking instant fire its easy to get stuck in a FOB if snipers are watching every exit, this is why its crucial to establish that perimeter.)
First guide figured id keep i short and sweet, ill try to update it into full on FOB guide, when i get mroe free time.