I suggest doing the calculations manually so you will fully understand what's going on when you use the available software. The basic calculations are simple multiplication and division. This is about 5th grade math more or less.
FWIW, here's a few tips
1. You must know grain bill weight
2. you must accurately measure the volumes
3. You must accurately measure the wort gravity
The most straight forward method is to measure the pre-boil volume and gravity. You then compare this to the theoretical max yield. The yield varies a bit from malt to malt and with various adjuncts. The yield is given as pppg (points per pound per gallon). The points are specific gravity points. Typical two row will yield about 36 pppg @ a theoretical 100% efficiency which almost no one can achieve. What you do achieve is what you will be calculating.
Here's the basic math stuff:
wight of grain x pppg = gravity units
ie, 12 lbs x 36 = 432 GU's
Your theoretical max yield from 12 pounds of grain will be 432 gravity units. You can divide this GU number by any volume and you can predict what it's gravity will be. For instance, using five gallons you would have: 432/5=86.4 or in the SG form 1.086. Six gallons would give you a wort with an SG of 1.072. Remember though, that these are max theoretical yields.
Now, you back into the numbers to get your actual pppg yield. Lets assume you have 6.5 gallons pre-boil with a gravity reading of 1.050. Your gravity unit yield would simply be 6.5 x 50 = 325 GU.
Your lautering efficiency is the ratio between your yield and the max theoretical yield. In the example this would be 325/432 = 0.75 or x 100 = 75% efficiency. You can also use the gravity unit yield to figure your post boil OG. You simply use the 5 gallon planned batch size at the end of the boil or whatever it is. You have 325 gu's available. Boil it down to five gallons and it will be 325/5 = 1.065 OG. Boil down to four gallons and you wil get 325/4=81.25 or an SG of 1.081. You should be able to see how the gravity goes up as you concentrate the wort to a smaller volume. The key is understanding the concept of gravity uniits or GU's. Understanding this basic brewing math comes in very handy. A pocket calculator is your friend.
