When you are in the Entrance Room of your base, a pop-up window listing three choices appears: Upgrade Plot, Add Personal Item and Edit Base.
Your plot is the grid that you place all your rooms on. You start with the smallest plot available.
To get a larger plot, use Upgrade Plot. Pick the grid you want. Keep in mind your current base has to fit entirely within the grid. You can upgrade (or downgrade) your plot at any time.
Each plot size has a maximum number of each item type that can be placed in it.
This is listed in the window on the right. Be sure to choose a plot that will let you place the things you want and take care not too spend too much. You will need Prestige to place rooms and outfit your base after all.
Add Personal Item
This button is grayed out until you build an item in the workshop. You build items with your salvage and place them even if you do not have base editing privileges. This way all members of your supergroup can contribute to the base.
Once you have built an item, you can use Add Personal Item to open the base editor. It will display all the pieces you have built. You can then place the item using the same methods explained below.