Everything downstream depends on this, so do it before you trace.
Three surfaces: tools down the left, options for the active tool in the floating box you can drag anywhere, and whatever you select in the right hand panel. Canvas setup sits under Settings.
Import layout puts your floor plan under the grid. Fade it with the Layout tool.
Set scale gives you three ways to calibrate: two known points, the overall width of the sheet, or the paper scale it was drawn at plus the image DPI.
Pick a grid module that matches your work: 100 mm for general, 300 mm for tile, 600 mm for cabinetry. Major lines every 10 modules.
Wall tool: click a corner, click the next, and the wall appears immediately. Keep clicking to chain a run, Enter or double-click to stop. A single press-and-drag draws one wall on its own. Type a number mid-run for an exact length.
Layout tool drags the sheet: grab a wall corner and it drops onto the nearest grid line. Straighten a skewed scan from any two points that should be level.
Right-drag pans, F zooms to fit, so you can always get back. Tab flips to 3D, then export a PDF with a graphic scale bar.