The GIF above shows how easy and simple it is to create an Altar, register blocks as "portal frames" and shortly shows how to edit the path of the dragon (although very incomplete).

Going from left to right:
The wooden sign lets you choose a name for the Altar, it's just there for identification.

The armor stand lets you specify the center of the altar. That will be the place where the dragon will be spawned. It actually does not have to be near the portal frames and can be everywhere you like.

The portal frame lets you specify portal frames which will be used later on to actually spawn the dragon. You can select how many you like, but it must be at least one. To select a portal frame left click it, to remove it right click the end crystal. To exit the portal frame editor, type "path:exit" in the chat.

The map allows you to edit the path the dragon will follow. Here is a short video on how to do that , one with a better explanation will be out soon.

To create a dragon, all you need to do is a few mouse clicks. The gif above shows how to do it.

Going from left to right:
The wooden sign allows you to change the name of the dragon (which is also shown in a boss bar). Use "&" to use color codes.

The heart allows you to set the health the dragon has. Note that it has to be lower than "settings.attribute.max-health" in your spigot.yml. As the dragon takes damage, the change will be displayed in a boss bar as well.

The chest allows you to set the loot the dragon drops once it dies. The loot follows very similar rules as specified here.
Options you can adjust:
Weight: the weight of the drop, read the article linked above for more information
Drop probability: the probability it drops (calculated once for every player that meet the requirements, see article above)
Depend on placed eyes: if set to true, multiplies the drop probability with the amount of eyes you placed
The actual drop: to change the drop, click any item in your inventory
Major loot: whether a drop can be given out only one time or multiple times

Lastly, you can edit the probability of the dragon being summoned. Note that it is not in percent but the specified probability divided by the total probability of all dragons that exist. For example the value for dragon A is 1 and for dragon B 2. The probability for A to appear is 1/(1 + 2) = 1/3, and for B to appear 2/(1 + 3) = 2/3.

You can edit which items are recognized as summoning eyes (can be multiple as well).
A summoning eye allows you to actually summon the dragon. You need as many of them as the amount of portal frames specified. To retrieve a summoning eye right click any portal frame.

ILibrary: https://www.spigotmc.org/resources/ilibrary.89796/
NBTNexus: https://www.spigotmc.org/resources/nbtnexus.111041/
Citizens: https://ci.citizensnpcs.co/
ProtocolLib: https://www.spigotmc.org/resources/protocollib.1997/