There are many different ways to close window blinds, and the best one to use varies depending on what type of blinds you own.
For horizontal blinds, you should start by pulling both ends of your blind toward each other to form a long rip. Next, tie the two bottom corners together with an overhand knot if possible, or simply use string or rope to hold them in place. If you can tie into the knots at top then do that instead as it will keep the loose strings from hanging down once you are finished.
For vertical blinds, the cord loop method is widely considered to be the easiest and most effective way to close them without tearing or breaking your blinds. This involves using a thin piece of strong string and carefully threading it through each strand of the vertical blind slats before tying them into a knot at both ends.
The best way to close these types of blind is by taking hold of both ends of their valance bar and bringing them together so they overlap. Then, slowly pull the valance down toward the bottom of your window until you feel a firm resistance. Once this point is reached, tie the bar in a knot and cut off any excess length with scissors.
Wooden blinds are probably the easiest to close without causing any damage. They should simply be pulled tightly together and then tied in a knot at each end using strong string or rope. These can either be left hanging from your window frame, or tucked behind the next slat up before being knotted at their ends as well for a neater appearance.
This type of blind is probably trickier to close than any other because they are typically long and made up of many individual slats. The best way to close these types of blind is by pushing all the slats against each other before folding the whole thing up at the top and gently coaxing it closed with your hands. Note: If one of your blinds is made up of many individual strings, such as a Roman Blind, then you may need to tie them together in order to prevent each string from unraveling when you try to close the blind all at once.