Step 1. Build your blueprint (I suggest making a copy of the finished product first).
Then go to the Blueprint Library (F6). Click the Steam workshop bar at the bottom.



Step 2. Click create new Item with the blueprint you want to upload Selected


Step 3. You will be automatically subscribed to the item you create, which you can't edit and make changes to. View your item in the workshop to make sure it's complete.



Step 4.Edit the description and title of your blueprint.



Step 5. If your blueprint requires any other blueprints to work (spawns in an item) then you need to make sure that the user has that item too.