Save a design once it works
Main Sequence supports saving ship and station designs for reuse and sharing. Save a design after you have tested it in the role you built it for. The official Steam description of Main Sequence lists design reuse and sharing alongside modular shipbuilding.
Keep a usable baseline
Save the version that has already done the job. For a cargo ship, that could be the build you have taken through a working pickup and delivery. If you decide to change its cargo layout, keep the proven version as a baseline so you can compare it against the revision.
A clear name helps if you keep several designs. Use the job and the difference that matters to you, such as an exploration variant or a hauler revised to carry a larger load.
Share the layout separately from your progress
A blueprint is the build you want someone else to reuse. Your active save is the world and progress you want to return to. Keep those purposes separate when deciding what to preserve or share.
Steam confirms design sharing. For the exact sharing controls, use the options in the build you and your friend are running. The store description does not identify a specific export format or distribution service.
FAQ
Can I save a station layout too?
Yes. The official modular-building description covers both ships and stations.
When is it worth saving a design?
After a test confirms it does what you need. Saving that baseline makes later changes easier to compare.
Does a shared blueprint transfer my campaign?
The announced feature is design sharing. Keep your campaign save separately if you want to preserve progress.