This is a hard question even by elite Minecraft players. There are two ways to find them, both hard. One is to go in caves until you hear silverfish sounds. This is super hard cause there are hundreds of caves in the world. Another one is the end portal version but is extremely hard. First you need ender pearls and access to the nether. You need to kill blazes to get blaze rods to make blaze powder. Combine ender pearls and blaze powder to make eye of the ender. Make 30 of them because they can break. You also need a boat because sometimes it goes into water. Do this until the eye go underground.Make a tiny 2x2 hole. Then you found a stronghold.