Here, you can find a short summary about waterproof treatments from GORE-TEX:
It is time to restore waterproofness when you notice that the exterior isn’t so water repellent anymore, i.e. water doesn’t bead up.
To start your treatment, first wash your apparel according to care instructions and then air dry it or dry it in the dryer on a gentle cycle.
As soon as the item is dry, put it in the dryer for 20 minutes to reactivate the DWR waterproof treatment (Durable Water Repellency) that was applied during manufacturing.
Instead of a dryer, you can also use an iron: To do this, turn it to a low temperature (warm, no steam) and put a hand towel between the apparel and the iron.
After a certain period of time, the treatment applied during manufacturing cannot be reactivated anymore. In that case, it is recommended you apply a new DWR treatment from a specialty brand on the outer layer of your item.
Here you'll find a tutorial.