After a loooong hiatus from reading the group, I just ran across this post.
If the towels and the floss are all cotton, they should be fine to just toss in the wash machine & dryer. Or wash, then iron on the back side to make the stitching stand out a bit more. If you don't know the brand of the floss, you might want to wash them in cold water. An oooooold standby is to use Orvus, which is really a livestock soap, but is gentle and safe for stitching. I have a gallon jug (only way it comes) and have only used a small amount of it, so if you'd like to try some, I'd be happy to share. Just email me privately with Orvus in the subject line.
Good to be back on the group! I retired last year so haven't been on the 'puter much...DH (Dear Husband) is keeping me too busy with his honey-do list! LOL