When you launch the game (but before you actually start the game) go to the extras/tool menu, and click in the catalog manager button. If you are using Windows you may need to hit alt+tab afterwards to see the box that pops up. From this box, you select the ship from a list of all the ships saved to your universe. Click on the one you finished, and then click export.
Then open a web browser, go to the docks and sign in, hit the community content page, then hit the button at the top of the page that says "add content"
Your exported ship will be in the starmade/ blueprints/exported folder in your starmade directory. The file you need will end in ".sment"