Easy. Water goes into one end (usually the left) and the pump sits at the extreme right---or vice versa. In one end, out the other. SKimmer resides in the middle. Heater in the middle. No bioballs, nuthin', just space for water. If you have a lot of space you can install a fuge in the middle.
To determine how much water, best to FILL the new tank from the sump. Water goes in, pump it up to the tank, stop when pump runs dry, add more water, over and over, until for the first time, water pours down from the top. Give it a couple more gallons, and that's it. Theoretically there will be 5-6" of empty above that waterline. That means if the power goes out, the sump can hold all the draindown from the tank above, everything in the downflow box to the top of the drain pipe, without flooding your floor.