I don't think I would go to all the bother of felting the wool to use as mulch mats - just spread it where you want it. If the fleece is already cotted then you will be using felt anyway, The dirtier it is the more feed the plants will get from it when it rains. I have tried this in the past but I don't think it keeps the big slugs away, and this year it's been the giant black ones which have killed off most of my veg (I didn't use mulch this year though). If you are determined to make felt for the plants, then just putting the whole fleece into the washing machine should make felt which you can then cut into smaller pieces.