Updated: Jun 8, 2020
When it's time to wash your garment run a line of strong thread up each side of the garment. Then soak it in warm water with some hair shampoo or Woolmix. Rinse it out and then squeeze dry in a towel.
Lay it flat on a towel on a soft surface, e.g. your bed, a carpet or interlocking rubber mats. Stretch it to shape and size (this is where the lines of thread will help) and pin it into shape. Leave it to dry – the longer the better – I leave mine for 24 hours. You'll find a photo of a drying garment included here – it illustrates the way to use the string and pins to help stretch the garment.
You can use a steam iron on the garment while it is damp if you wish. This works quite well to restore the shape.
If you find you're not confident about washing the garment talk to an experienced knitter, or put ‘blocking’ in a search engine to see various ways of bringing the garment back to its best.
If all else fails, consider sending it back to me and I will wash and re-block it for you – for the cost of return postage.