1.   How can I warm the basement without making vents in the ducts serving my first-floor unit?

2.   If there are not enough windows or none at all, make some vents in the wall.

3.   Then you make some vents.

4.   To make the vent, you have to remove the shingles or clapboards and cut the sheathing where you want to work.

5.   To vent, make some vents in the roof overhang, if you have a roof overhang.