Players can re-summon the dragon by placing four end crystals on the edges of the exit portal, one on each side. When it is re-summoned, the four end crystals.
To enter the end you are going to need to go on a search for a stronghold which is usually underground. This can make finding the portal a challenging prospect.
There's no real way/method to find a nether fortress, you just have to wander around until you see one. They do seem to spawn mostly in or near big lava lakes.