There are two ways to make this: 1) A latch that can be used for doors with broken or missing doorknobs. This latch will keep your door closed, but not necessarily keep people out. However, small pets and insects will be thwarted in their attempts to get in (provided the door is properly sealed). 2) A latch that will be more permanent and will keep your door locked while you are inside.
For a lock:
Aluminum Can Scrap.
Super Glue or Epoxy.
Small Metal Loop or Plastic Zip Tie.
Soda Bottle Lid.
Small Metal Rod (about the width of a pencil)
For a latch:
Plastic Zip Tie.
Soda Bottle Lid.
Pencil or chopstick.
Latch Version: Take the knife and twist the blade into the soda cap until there is a hole in the center of the lid that is just large enough to fit a doubled-up twist tie snugly.
Lock Version: You can either keep this step and the next step the same, or you can simply epoxy/super glue a small metal loop (just big enough to fit the metal rod/pencil snugly) to the middle of the soda cap.
Latch Version: Wrap the twist tie around your pencil/chopstick and pull the twist tie around it tightly. Then take the twist tie off of the pencil/chopstick. This loop will be used in the next step.
Lock Version: (You can skip this step if you chose to glue the metal loop onto the cap. If you chose to do it with the twist tie, then follow the step above.)
Push the loop (smaller end first) through the hole in the cap. Cut the long end of the twist tie off if it extends beyond the length of the cap. Getting the twist tie loop through the cap may be easier if you crease the loop.
Cut a hole in the center of the fabric (or the aluminum can scrap). Take the cap (with the loop on it) and thread the loop through the hole. Take the chopstick/pencil/pole and stick it through the loop. You're almost done!
Go into your room and close the door. Take your thumbtacks (or nails) and secure the fabric (or aluminum scrap) to the door. If your door opens inward, then secure the latch to the door frame. If your door opens outward, then secure the latch to the door. To latch your door closed, simply twist the pencil/chopstick so that it is horizantal. To unlatch, twist it 'till it's vertical.