Snoots are used in theatre alot (also called top hats) and are used to prevent light spill and ensure that the only light seen is that reflected off the target, not that from the lamp itself.
Snoots are very useful in the bedroom to allow one to read while not disturbing the other occupants with "light pollution".
Step 1: Measure twice cut once.
-Cardboard (black and thick, flame retardant) DO NOT USE NORMAL CARDBOARD!!!
Alternatively black foil could be used which is obtainable from theatrical shops.....
-PVC tape (Flame retardant)
--Cheap halogen bedside lamp that spills way to much light
Measure the outside of the lamps head (pic 2) and ensure card is long enough.
A piece of A4 should be sufficient for most lamps.
Also measure any protrusions or knobbly bits