Depending on your preferred craft, you can make a soap sack that is sewn, knitted or crocheted, and best of all, it takes little time with any method.
A sewn sack can easily be made by cutting an old washrag down to size and making a three-sided pouch to hold the soap. You can sew a button on to keep the soap in, or sew the top over and install a drawstring. Just be sure you can easily add a new bar when that bar is spent.
For knitting, follow instructions to make a pouch knitted in the round, or make a long rectangle and sew it together when finished. You can weave a bit of ribbon through the stitches for a drawstring.
Crocheted soap sacks can be made the same as a knitted one, with the benefit of larger holes for the soap to come through.
One thing to remember is that soap will be used up faster when it is in a soap sack, so you will start going through more soap. I found that the experience of using my own handmade soap sack was worth the little bit of extra soap. It looks much better than a traditional wash cloth, and makes it easy to keep your soap to yourself. You can make individual soap sacks for every member of the family!