Here's how:

Step 0: Check your blueprints on a game save and ensure that the blueprints are in a collection and that it is named something clear and easy to remember.

Step 1:Open up NIMBY Rails and on the main menu click Options. Make sure a game save ISN'T running or you'll get an error message.

Step 2: On the Options, click on Uploader on the bottom left, then click Use Mod Uploader.

Step 3: Once there, you should be able to click New Mod. For blueprints, select the Blueprints from the category dropdown box. Here you can fill in all the required information and select the blueprints to upload.

Step 4: Once all filled out, press upload mod and there you go, uploaded on the workshop!

Hope that helps :)