What many of us do is pickup a good 30 gal
plastic trashcan. You could use one of those 20 gal
clear totes if you wished. As long as it does not have any holes in it. Fill this with water. Add a heater and a powerhead or two. Let the water come up to tank temperature and then slowly sift in salt till you have added approx the right amount to get the SG
up to the level that you keep your tank at. Losely cover and let sit overnight. Check the SG
of the water and if its in line with your tank then your ready to use the newly mixed water.
You dont want to add fish till ammonia and nitrite are zero. Unless you leave the tank empty for a few weeks at zero ammonia after the shrimp cycle you wont have to worry much about the bactera dieing off.