How do you build from a projector?

[Riku]

i have a BP projection on but no way to start building it whatever way i set the projection, nothing O_O i don't understand

[Laef]

Open the projector in the control panel, adjust the projections location until one of its blocks touch the projector. You will then be able to weld the blocks in contact with the projector.

[Nightwing]

The blocks don't actually have to directly contact the projector. So long as the blueprint makes contact with the grid that the projector is on, that's all you need. It's useful if you're trying to mass produce ships, since you can put the blueprints on a sprue, which can be cut as soon as the ship is built.
For those wondering, a sprue is a line of blocks that extend from a blueprint solely to connect the blueprint to a grid from a safe distance. They're meant to be cut after the ship is complete. If you've ever seen molded plastic, and noticed a circle that may or may not have a little point in the middle somewhere on it, that's where the sprue was for that piece.

[DizzieDawgie]

I found this video very helpful to understand projectors:
https://www.youtube.com/watch?v=uLVnhk7wzoI
The vid is a year old, but I don't think much has changed since then.