How to automate blueprint building in space engineers?

Depends if you want to use mods or not:

If you wan't to do it without mods, then you could have a wall of welders that moves back and forth by a piston. Extend the piston as far as it can before starting the welders. When it's fully extended, turn on your projector block that contains the blueprint, then turn on the welders and retract the piston at steady but slow pace. It will construct the blueprint with each layer it passes trough. If the piston retract faster than your welders can weld, you could stop the piston let the welders finish welding the layer and turn the piston back on again.

However this takes a ton of resources to make such welding wall, and it will create lag if you play on a server.

So I rather suggest using a mod from the workshop like the "Nanites Control Factory" which I use.