Silage is made either by placing cut green vegetation in a silo, or by piling it in a large heap covered with plastic sheet, or by wrapping large bales in plastic film.

Silage must be made from plant material with a suitable moisture content, about 50% to 60%, depending on the means of storage, the degree of compression, and the amount of water that will be lost in storage. For corn (maize), harvest begins when the whole-plant moisture is at a suitable level. For pasture-type crops, the grass is mowed and allowed to wilt for a day or so until the moisture content drops to a suitable level.

The plant material is collected, chopped into pieces about 0.5 in (1.3 cm) long and packed. In the early days of mechanized agriculture, stalks were cut and collected manually using a knife and horsedrawn wagon, and fed into a stationary machine called a "silo filler" that would chop the stalks and blow them up a narrow tube to the top of a tower silo. Current technology uses mechanical forage harvesters that collect and chop the plant material, and deposit it in trucks or wagons. These forage harvesters can be either tractor-drawn or self-propelled. Harvesters blow the silage into the wagon via a chute at the rear or side of the machine. Silage may also be emptied into a bagger, which puts the silage into a large plastic bag that is laid out on the ground.

In North America, Australia, North-Western Europe, and frequently in New Zealand, silage is placed in large heaps on the ground and rolled by tractor to push out the air, then wrapped in plastic covers held by recycled tires.

In New Zealand and Northern Europe, the silo or "pit" is often a bunker built into the side of a bank, usually made out of concrete or old wooden railroad ties (railway sleepers). The chopped grass can then be dumped in at the top, to be drawn from the bottom in winter. This requires considerable effort to compress the stack in the silo to cure it properly. Again the pit is covered with plastic sheet and weighed down with tires.

In an alternative method the cut vegetation is baled, making balage (North America) or haylage (UK).[clarification needed] The grass or other forage is cut and partly dried until it contains 30–40% moisture (much drier than bulk silage, but too damp to be stored as dry hay). It is then made into large bales which are wrapped tightly in plastic to exclude air. The plastic may wrap the whole of each cylindrical or cuboid bale, or be wrapped around only the curved sides of a cylindrical bale, leaving the ends uncovered. In this case, the bales are placed tightly end to end on the ground, making a long continuous "sausage" of silage, often at the side of a field. The wrapping may be performed by a bale wrapper, while the baled silage is handled using a front-loader, either impaling the bale on a spike, or by using a special grab. Holes made by a spike are re-sealed to avoid spoilage.

Silage undergoes anaerobic fermentation, which starts about 48 hours after the silo is filled. In the past, the fermentation was conducted by indigenous microorganisms, but, today, some bulk silage is inoculated with specific microorganisms to speed fermentation or improve the resulting silage. The process converts sugars to acids and exhausts any oxygen present in the crop material. Fermentation is essentially complete after about two weeks. Silage inoculants contain one or more strains of lactic acid bacteria, and the most common is Lactobacillus plantarum. Other bacteria used in inoculants include Lactobacillus buchneri, Enterococcus faecium and Pediococcus species.

The fermentation process of silo or pit silage releases liquid. Silo effluent contains nitric acid (HNO3), which is corrosive. It can also contaminate water courses unless collected and treated – the high nutrient content can lead to eutrophication (growth of bacterial or algal blooms).

Plastic sheeting used for sealing pit or baled silage needs proper disposal, and in some areas there are recycling schemes for it.

Silage must be firmly packed to minimize the oxygen content, or it will spoil. Four major stages silage goes through in a silo: