The side of the container has mixing ratios on Red Sea, atleast does in europe. I weigh my salt (use a small kitchen digital scale). (38.2 grams per liter).
Depending what size tank you have, find a suitable container. I don't have a 'mixing station' nor space for large brutes. For my first tank a small 'mop bucket' from local 'walmart' type store let me mix a 15% water change. With my new tank (still small to many but massive to me), I found a larger bucket at the local hardware store.
General routine is:
Make RODI Water (Hooking RODI unit to the shower - Make sure it's on cold before connecting)
Add heater + pump
Mix in salt by weighing it.
I usually do this on Saturdays, and let it mix overnight, then do the water change Sunday.
Always do a sanity check using the refractometer. Checking both the tank and the newly mixed water.
I use a second identical bucket to drain same amount out, then put the new water in.
I used to just pour it in, but now use a small tunze return pump + hose to pump the water in. Much nicer than pouring by hand.
Do you have an RODI unit? It's probably fair to assume your LFS used one to make the SW you were buying. Depending what's in your tap water, you may(probably will) run into issues using it straight from the tap.
So a summary of 'parts' needed:
0.) Unit to make RODI water, or buy RODI water.
1.) Two buckets of equal size. One to mix, one to drain into. Can also stack them when not in use.
2.) Pump of *any* kind to mix water. (I've used power heads, return pumps, etc whatever is around)
3.) Heater set to right temp
4.) Hose to syphon water out
5.) Refractometer
6.) (My preference) a kitchen digital scale to weigh salt