Use the leftover whey from making another cheese.
Return the whey to a pan and re heat to 90 centigrade/ 200 Fahrenheit and hold at temperature for a couple of minutes. Turn off the heat and leave to cool, to 60 centigrade/ 140 Fahrenheit. While cooling small curds will begin to form.
Line a colander as before in a sink, but don't worry about catching the whey this time, and pour the whey into the muslin to drain.
Fold up as before and to strain as much moisture from the curds as you can.
Pour the curds into a ricotta basket, or equivalent, and leave to drain, cool and firm up.
Turn out of your basket and store or use.