You are going to get wet, you are going to mess up and get tired. It is best to learn in warm, still water with warm air as well. I love to practice in an indoor swimming pool in the company of friends, preferably highly skilled rolling friends. Many cities and towns have local paddling groups that will rent pools during the off seasons for pool sessions, some also organize training sessions devoted to specific rescue techniques, see if you can join your local one, it will be a great way to practice learn and improve. If there are no groups to join then ask local paddlers where they practice, you are looking for calm sheltered water free of currents and waves and preferably not much more than chest deep.