Machine wash separately in warm or cold water on a gentle cycle.
Use natural/gentle laundry detergent.
Line dry in shade if possible to avoid bleaching by the sun.
Can be tumble dried on a very low heat - do not over-dry.
Things to avoid:
Bleaches or Whitening Agents as these may leave discoloured patches on the towels. These patches, at times, may not appear until after washing. Bleaches will also weaken and break the natural cotton fibre over time.
Fabric Softeners, most will put a chemical coating on your towels and make them less absorbent.
Pulled threads should be cut off at the base of the towel. This will not effect the towels use or lifespan.