You can put the ramekins into sealed bags, but I found it very difficult to keep the custard mixture from spilling over or keeping them flat during the sous vide process. What I recommend instead is to find out how many liquid ounces your ramekins hold and use a sealable jar that will hold that much mixture. Sous Vide in the jar and then once it's done, immediately pour it into the ramekin while it is still very soft and liquidy. Place in the refrigerator to chill. I recommend covering the ramekins after about 15 minutes in the fridge so the top stays smooth and doesn't develop a skin on top.