Purchase a ball of thick yarn in the color you would like your rosary to be. The yarn should not be fibrous or easily frayed, but tightly spun and strong.
Unravel the yarn so that you have approximately a 1 foot string loose from the ball of yarn. Do not cut the string. Hold the yarn about 6 inches from the loose end. Make a knot in the yarn and repeat knotting in the same spot three times, pulling the string taut each time. You should now have one large knot. If the knot is not large enough to easily feel with your finger, repeat the knotting in the same spot one to three times more.
Make a second knot approximately have an inch away from the first knot. Repeat knotting in the same spot with the exact same number of knots you made in the first one. Continue making the knots at equally spaced intervals on the string of yarn.
Complete as many knots in your yarn rosary as you desire. Rosaries can range from a count of 12 to 99 or more. Separate the completed yarn knot rosary from the ball of yarn by cutting 6 inches away from the last knot.
Tie the two ends of the rosary together. Add a yarn tassel or an ornament by tying it to the end of your rosary if desired.