First thing, you need a hydrometer. This will measure the salt in your water.
Any fish store will sell the plastic swing arm type for cheap.
If you are willing to spend around $50, you can get a more accurate one called a refractometer.
You can find examples of both
here.
After that, go to some place like home depot and pick up a big rubbermaid type trashcan that holds like 30 gallons.
Roll that up near your tank and put water in it.
Then mix salt with something like the plastic handle on a algae scrapper. Each bag of salt will say how many gallons of saltwater it will make. So mix enough to get you around the same as the water in your trashcan.
After mixing well, use your hydrometer to get you near 1.024 - 1.026.
If you want to be really good, put a heater in the trashcan and a powerhead or air stone and let sit overnight or so.
Or you can mix in a small bucket, but it will take a lot of buckets to get to 75 gallons.
Or you could mix in the tank if nothing else is in it and add the sand and rock after the salt water is well mixed, but adding the sand and rock later can be very messy.
If you are using dry sand, I would suggest rinsing it in a bucket first to remove debris.
Hope that helps! Thanks for joining!
Matt
