Each 0 or 1 has a numerical value that is computed when calculating the 1's, which the slots are:
128 | 64 | 32 | 16 | 8 | 4 | 2 | 1
If you subtract 128 from 255 and you get a positive number, then you put a 1 in the first column; otherwise you enter a zero. Then go down the line. From the new value of 127, you subtract 64 and if you get a positive number, you put a one in the 64 column; otherwise, enter zero.
And so on and so forth.