This How-To is best used when you've got a basic floor plan in mind.
To start off, select the terrain tool, and level the lot. This way it hopefully prevents uneven terrain spots as you're building.
Use the free wood flooring or any other flooring to flesh out the floor plan the way you want it.
Tip: Use a different colored flooring as spacers for the different rooms you want in your floor plan and for where you want to put the stairs.
Tip: Use walls if desired, to get a better idea of where you might want the staircase to go.
** If walls are used, they may need to be deleted before stairs can be placed.
Tip: To remove the roofs the rooms make select the roof icon and turn off Autoroof. Then use the Remove All Roofs button.
Using the basement tool, Make the stairwell to basement a little longer and wider than where you want the stairs to be, to start. In this case, 4 squares to the front and 2 squares to each side. In total I made a 6x9 basement to place the stairs in.
Doing this makes sure you have enough routing space on all sides of the stairs when they get placed.
Tip: Leave a little extra room for the stair landing - from the foundation to landing, then landing to basement. (6-9 squares in-total for stairs and landing, plus room around stairs.)
[Stairs = 7x2 to help with landing.]
Remove all the walls if used before placing stairs, but leave the free flooring as a floor plan guide. The reason being is that stairs won't place with walls being in-place.
I set Auto-Railings off, so I can place my own later, and Stair Width to 2. (Use the Arrow keys to rotate stairs.)
Pick the stair style you want, then place them. In this case, I placed them 2 squares away from the walls in back and front. This allows plenty of room = no routing issues.
I usually check to make sure I've placed the stairs correctly, by going up to ground level and check placement.
To place the foundation, select the foundation and hold the left mouse button down, while dragging it. To delete mistakes, press the left mouse button and Ctrl together while dragging it.
You can place foundation over the place where you want stairs to go upstairs. But don't cover up the landing or staircase for the basement!
Tip: To make edits to the foundation, use the hand tool and hold the mouse pointer over the foundation you want to move, and push/pull the arrow to edit the foundation. You can do the same with the walls.
Draw out the walls, place the stairs in the landing and to the 2nd floor, make room and foundation adjustments as desired/needed.
Use Left-mouse-click and drag to move the foundation and walls.
Hold the Left-mouse button down, while using the hand tool to drag the foundation in or out. You can do this with walls as well.
Build the rest of the basement by going to ground level and selecting the basement tool to draw out the rest of the basement. Think of it as coloring the inside of the foundation Blue.
Use the floor down button to get to ground level. I usually use free flooring to cover the ground under the foundation.
To make edits to the outer basement walls or to delete parts of the basement, go to ground level using the basement tool - hold the left-mouse-button and Ctrl key down and drag to delete outside walls.
-- Once you're finished with the basement, you can finish the rest of the house.