Founding a new base
Send a Construction Supply Convoy somewhere new and build a base on the spot.
Any base can order a Construction Supply Convoy, though not more than one a day. It joins one of your army groups near the base, or forms a group of its own.
Take the convoy to its new home, stop, and press Found a base here in the group's panel. The new base starts small, and the convoy is used up.
A new base needs plenty of room: it must stand on land, well away from every other base. While the convoy is selected, the map hatches the places that are too close in red and points to the nearest open site.