Seeing as there are so many questions about Battleforged on here and other forums, I thought I'd write a short guide to what it means and how to do it.
Battleforged means that all the units in your army are part of a detachment or formation. Ino rder to compensate for being limited in this way, if your army is battleforged, you get command bonuses, which are different for each detachment or formation.
In the brb there are two types of detachment - combined arms detachment (CAD) and allied detachment. CAD is the usual force organisation chart (FOC) and requires 1 HQ and 2 troops. Your troops get a bonus to hold objectives if they are part of a CAD. The allied detachment allows you to take units from a different army and requires only 1 HQ and 1 troop choice. They get the same bonus. Other unit types are available for these detachments.
You can, as an example, have an army picked from Codex: Space wolves as your CAD and units picked from Codex: Space marines as your allied detachment (both adhering to those FOC). This is a battleforged army.
You can also include formations in your army. Formations are specific to each faction or codex. For example, there is a formation in the new Eldar codex that has three units of dire avengers, which gets bonuses to shooting. As long as your army contains nothing but formations and/or detachments, it is battleforged. For example:
Avenger shrine formation by itself = battleforged army.
Three avenger shrine formations = battleforged army.
Three avenger shrine formations and a crimson hunter = not a battleforged army, because the crimson hunter is not a formation or part of a detachment.
You can mix detachments and formations; for example, here is a battleforged army:
CAD (space wolves)
1 rune priest (HQ)
2 x 10 grey hunters (troops)
3 Thunderwolf cavalry (fast attack)
Formations (eldar)
Seer council, 2 farseers, 5 warlocks
Aspect shrine, 6 fire dragons, 5 dark reapers, 10 dire avengers
This army has units from different factions (elder and SW) so we would need to use the allies matrix in the brb to see how they interact with each other.
Hope this helps!