Lay the canvas bag down flat. Use the paint brush to brush the black paint onto your child's hand. Don't paint their thumb - you only want the 4 fingers for this craft. (We found that handprint crafts turn out a little better/"cleaner" when we painted our daughter's hand than when she dipped her hand directly into the paint).
Help them carefully lay and press their hand onto the bag and then pull straight back up so the handprint doesn't smear.
Either turn the bag and paint the same hand again, or paint the other hand with black paint. Press the second handprint down so that the palm of it overlaps the palm of the first handprint.
When the spider has dried, use red paint to add a smile. Glue on the two googly eyes. Paint a line coming out of the top of the spider so it looks like he's hanging.
Use the black paint to write "Happy Halloween!" onto the bag. **Note: although you are painting on fabric, regular black craft paint is ok for this craft assuming it will most likely only be used on trick-or-treat night and won't be washed. If you think your craft will get more use, please use fabric paint instead so that the bag holds up better.