The way to ensile a small portion is simple.
5 gal. bucket w/lid. Place a black plastic bag inside. Pack material you want to ensile inside as tight as you can. Twist the bag closed so that it is sealed from air, put the lid on the bucket, in 2 weeks (weather dependent) it will have ensiled. Corn silage needs to be about 65% going in. Not sure on pure grass. Most people dry grasses, alfalfa, etc instead of ensiling them.
If you take out what you want, then pack and reseal, you should lose very little to none. Now if you leave it uncovered, be prepared to lose it all.