1. Open Lighting → Clouds.
2. In Preset, choose a sky, for example Scattered Cumulus. The first time, Sitrec makes the noise textures that shape the clouds. Status shows when they are ready.
3. Look at the sky in the look view. Clouds are drawn in the look view only, unless you tick the M box on the folder's title row.
4. If the ground is high, for example in the mountains, raise each layer's Base Altitude by the height of the ground. The default altitudes are above the WGS84 ellipsoid, not above the ground, so they suit a place near sea level.
5. To change the clouds, open the layer's folder (for example Layer 1: Cumulus) and change its settings.

The clouds are saved with the sitch.