When you realize you are ruminating acknowledge it. Then return to the present moment and what you are doing. When you notice thoughts again... don't engage, don't do compulsions... just yep hey ocd thoughts or thanks brain for those thoughts and then... return back to the present. Just let the thoughts be there, feel the discomfort and continue on with your day... When you first start this you'll have to do it a lot in a days time... but it will get less and less because you're doing ERP!