Actually, when adding or subtracting in any base, you can follow the same mechanics as those used in base 10. In your problem 100 - 11, you just have to remember that 1-1 = 0 and 10 - 1 = 1.
.100
-011
You have to do some borrowing. You borrow 1 from the leftmost digit in the minuend leaving it a 0. This makes the middle number a 10. But we have to borrow 1 from it making the middle number 1 and the rightmost number 10.
Now reading from left to right in the minuend the first column is 0, the second column is 1 and the third column is 10.
Now, do the subtracting. Starting at the rightmost column, 10-1=1. The rightmost number in the remainder is 1.
In the middle column we have 1-1=0. The second digit from the right in the remainder is zero.
In the leftmost column we have 0-0=0. Our remainder is 001 or simply 1.