Sardines are saltwater fish that attracts almost any kind of saltwater predator. If you want to use them live, catch them by chumming the water and throwing a cast net out to collect a bunch. Keep them in a live well with plenty of aeration and constant fresh seawater.
Sardines are suitablefor freshwater fishing, but only as non-live, frozen bait. If you purchase them on ice from a bait shop or the grocery store, look for blood in the sardines' eyes to show that it is fresh, and ask to ensure that they were flash frozen. This is the quickest way to freeze something and avoids tissue damage in the fish by doing so. Cut them into fillets or chunks, or bait them as whole.
For salmon and steelhead fishing with sardines, some fishermen wrap sardine fillets around plugs, which makes the whole bait system extra enticing for the predator fish. Plugs are hard, artificial lures that look like fish, so sardines add scent. To increase the scent and strength of sardines, brine them overnight. Make a brine using sea salt and olive oil, and pack sardine fillets in this for 10-24 hours.
One of the difficulties of fishing with sardines is that smaller fish will be attracted and may pick away at your lure. You can avoid this by using chunks, or even using the skeleton or head of the sardines as bait instead of the fillets.