Define a cross-section plane
- Launch the Cross Section window
from the sidebar and activate the Plane checkbox. - To define the plane, click Set plane > Horizontal and click on a point in the viewport with the desired elevation.
- Press Create, and the plane will be added to the Object List as an individual plane object.
Trace the section with curves
- Activate Snap to Section
and Snap to CAD 
from the toolbar. - Then, launch the CAD window
from the sidebar. - Set the Drawing Plane to horizontal.
- Choose the desired curve type and start tracing the section.
- Repeat the previous step as many times as required.
Fit drawing to section
- Apply curves-to-section fitting as described here.
- If necessary, select two or more end-to-end touching polylines and press Join
to combine them into a single polyline. - To split a polyline into a sequence of lines, select the polyline and press Explode
.
Finally, export your floor plan into a DXF or DWG file. A step-by-step guide is described here.