Open Stormworks
Go to the main menu
Select the Workbench
Choose a save slot or create a new one
Open the vehicle or item editor
Click the Workshop tab or Workshop button
Browse or search for the item you want
Select the workshop item
Subscribe to or download the item if needed
Load the item into the editor
Place it in the world from the editor
Spawn the vehicle or object using the spawn button
If needed, enable addons or creative mode in the save settings