The reefs of the world are the most stable environments on Earth, despite human activity and pollution tipping that balance in the reefs of the Caribbean Sea and the east coast of Florida. The natural reef water possesses a set ratio of salts, minerals and trace elements. Temperatures vary little, and in most reefs, not at all.
Several different manufacturers product synthetic salts that posses the proper ratio of minerals and trace elements, but there are some brands that are superior to others in that respect. Seek out impartial reviews to give you an idea which salt mix to buy.
Natural Sea Water (NSW) contains 11 major elements and a handful of trace elements. To simulate that in our marine tanks one must make every effort to be as close to the natural world as possible.
Tap water is most usually quite unsuitable, so the majority of serious marine hobbyists have a reverse osmosis unit, and many add a dionization stage to help produce the purest water.
What you will need:
A Hydrometer, Refractometer (better) or electronic salinity meter (best).
A clean container based on the amount of saltwater needed.
A clean large spoon or similar.
Air pump and air stone.
Mag drive pump or similar.
Fill the container with the pure water. Then, start adding salt slowly, testing often. Most brands say X amount to X amount of gallons, but until you understand the correct amount of marine salt necessary for 1.026 specific gravity, which can also be listed as 35 parts per thousand (ppt), add marine salt slowly and test as you do it. Stir as you add the salt to mix it thoroughly.
Before use, use the heater to warm the water to that of your tank, and use the air pump to aerate the water and mix the warmth. After several hours while the heater and air pump do their work, use the Mag drive pump to pump the sea water into your aquarium.
Mixing saltwater properly is an elementary part of a marine aquarium, and a skill that must be learned.