Constructing and Deconstructing Walls
From Unofficial Stationeers Wiki
Getting the basic structure for your base is usually the one of the first things you do. You will need a base to install your solar panels, solid fuel generator and get the battery charger hooked up to an APC to get power from the solar panels. You need frames to attach these structures too.
What you need
Iron Frames are your basic building blocks. Iron walls can be used but they don't allow structures to be placed. Iron frames do not have to have iron sheets added on them to place structures. You can use the frame itself. The frame without sheets allows you to walk through it and place cables and pipes inside. Which you can seal up after to pass connections through rooms.
Grab Iron Frames into active hand, use right-click to bring up place mode and left-click when frame is in desired spot.
Then equip a welder in one hand and Iron Sheets in the other hand, two per frame you wish to install. The welder needs to be the active hand.
Use the 'O' button to toggle the welder on.
That completes one wall/ceiling/floor that you can place structures on.