Step 1: Select Objects palette on the palette tool, navigate to Repeatables, and select Barriers
Step 2 : Trace the walls and objects you would like to block light with barriers, they will appear as red lines. There red lines are only visible to the DM, not the players!
Step 3 : If you hold ctrl while drawing, the lines will allign to the grid, if you hold shift, you trigger free-draw, so you can trace uniquely shaped objects
Step 4 : If you need to delete barriers, use the eraser tool, and dont forget to tick barriers
Step 5: You are ready! Turn the time of day to night, and place a light source to check your work!