I USED to draw mi briks with an exacto, an then marking them really well with a pencil/pen.
But now I use a self made hot engraving tool, and my life is much more easier!
Just bear in mind that the fumes are toxic, so you MUST do this outside or in a WELL VENTILATED area.
Your work looks pretty good, and sure will be a pleasure to play on that scenery.
Best wishes.