This style in general, thankfully, did not survive the 1970s (except in certain locations). Here’s how you do it:
Fill a bathtub with warm water.
Add 1 cup of bleach to the water.
Knot the legs of a pair of dark blue jeans several times.
Place the knotted jeans in the water.
Let them soak for 2 hours.
Remove the jeans, untie the knots, and place the pants in the sun.
When the jeans are dry, you will notice that they now have large, unpredictably shaped pale patches where the bleach settled, giving your jeans the appearance of a pinto pony. Why would you want your jeans to look like a pinto pony?