Its easy. For hypothetical situation (since you did not leave any info on your tank) lets say it 100Gal, and your going to do a 25% water change that 25gal.
First go out to local HD or some other place and get 10 5gal buckets, go to LFS and get enough salt to make 30gal or more of water, and go to the store and buy 30+ gal of RO or DI water.
Make up your saltwater in 5 of the 5 gal using your new salt, and RO or DI water. Test its pH, temp, and salinity making sure its same or very close to what is in your tank. You may have to warm up the water a bit by putting hot water into some of the containers that your RO/ DI water came in and placing them in the buckets this will warm up the water. The pH should be around 8.2 and that’s about what your tank should be.
When your water is very close to the right temp drain water from your tank into the other 5 gal buckets. Try to skim from the top because this is were nasty stuff floats around. Once you have removed around the same amount that you have ready to put back in just slowly start to add the new water. Its much easer if you have a sump but you can poor it right into the tank just do it slow. If you need to make up more water due to spillage or what ever do it in the new water buckets.
When you first start this process it will take a while. As you get more and more water changes under your belt it will start to go by faster and faster. I try to do 5% - 10% every week so I don’t have to fool with large amounts of water.
