Blueprints are a great way to speed up the building process. With blueprints, you'll be able to create a template of a structure you want to create, and use it to replicate the same thing after the blueprint is saved. This can be a great time saver when building a large factory with repeating parts.
To being, you'll have to have unlocked Tier 4 and have unlock the Fiscit Blueprints Milestone. Once you have unlocked this, you'll be able to start creating with blueprints. Once you have met these conditions, open the Special Builder Menu and select the Blueprint designer, pictured below.
Once you have this selected, place it down in an open area where you have plenty of room to build. Now, you can build whatever you would like in this space. If you select recipes in the blueprint designer under certain structures to manufacture something specific, those recipes will be saved. Once you've finished building your blueprint, you can go to the Blueprint designer terminal. You'll be able to save your blueprint and organize it into a directory, giving it a name and icon.
Once saved, you can use the blueprint as an item by going to the Build menu, then selecting Blueprints, and putting your blueprint in your hotbar. Make sure you have the materials to create the blueprint, and you'll be able to place your custom blueprint structure wherever you need.