Linen wants simple treatment, and it turns softer the more you wash it.

Wash cold on a delicate cycle. At 30 degrees or cooler, inside out, with a mild detergent and no fabric softener, which coats linen and dulls its crispness.
Air dry, skip the tumble dryer. Dry flat or on a wide hanger away from direct sunlight. High heat shrinks the weave and breaks the fibre; if you must tumble, keep it brief and low.
Iron while slightly damp. A light spritz of cool water, the linen setting, and a pressing cloth to prevent shiny spots. Or leave the relaxed crease, which is part of linen’s character.
Store folded in a cool, dry place. Fold with acid free tissue away from sunlight. Long storage on a hanger distorts the shoulders.
Spot clean quickly, dab never rub. Cool water and a drop of gentle soap, treated within the hour.

The bottle for it

Made for this fibre, by a textile care specialist in Mönchengladbach.

For what this fibre is and where it is made, see Linen in Materials.