Well, your best bet is not to use tap water but rather purified water such as reverse osmosis. Tap water usually has too many impurities for a reef tank to handle. If you previously had someone do your water changes than I am willing to bet you don't have an RO unit, so you will need to go out and buy some pure water. Most saltwater fish stores will sell it by the gallon. Back when I had to go to the store to get mine I used NEW 5 gallon plastic gas containers for easy transport.
You want to mix the salt with the water for about a day or so with a powerhead, and then test to see that you have achieved salinity around 1.025. For a small change on a large tank I usually don't get all caught up in getting it perfect. (Also, many folks use a heater and match the temperature of the new water to that of their tank, again for a small change on a large tank I don't think this is a big deal, but if you are changing a large volume of water you probably want to stick a heater in and get it up to temperature) Consult the salt container for the approximate ratio of salt to water, I have been doing it so long I just know about how much to pour in now so I can't say how much to add.
I don't have any particular opinion on salt brands, I am sure someone else will chime in, I have been using Coral life, because it is what the store sells near me in convenient sizes.
Once the salt water is mixed, take a hose and siphon out from the tank the same amount of water you mixed up. If you have an auto top off system, be sure to turn it off first. Also unplug any heaters that may become exposed due to the drop in water line. When I siphon out the water I try and look for any pockets of detritus and get those out of the tank by means of the siphon.
Then, pour in the water you mixed up, and be sure to plug in any equipment you unplugged earlier!
Also, note that many saltwater fish stores sell pre mixed saltwater, you can just go buy some and buypass all the mixing if you like.