Spot clean and hang outside in sunlight where possible.
For best maintenance, wash slips at 30℃ and hang by the waist to dry. Avoid sleeping in the viscose slips as it may bobble against the cotton- polyester bedsheet.
Extensive washing at high temperatures and high spin will eventually result in fading, bobbling and stretching.
If you are someone who fires it in washing machine and hopes for the best- there's no judgment here ;) Do try turn the slip inside out and you can pull the slip back into shape before drying. In the event of bobbling from high spin, lightly use a razor to remove bobbles.
Follow the instructions on the garment's label. If the label is no longer attached, handwash and hang to dry to be safe.
(I generally place the item in a pillowcase and wash on the gentle wash but this is not an official recommendation.)