The longship is the upgrade of the Raft. To unlock the craft you need to defeat the Elder Greyward as it's the only mob to drop Elder bark.
How to sail a longship
How to build a longship
How to sail a longship 
Edit
Just the same as sailing a raft.
How to build a longship Edit
You need to a workbench near in order to place it down.
Ingredient:
- Wood x20
- Hide x10
- Resin x10
- Elder Bark x10
- Select Hammer that in your inventory
- Press [F] to switch to [Misc] tab
- Press [Q/E] to switch to longship
- Press [LMB] to place it in the water
